Based on https://nshipster.com/temporary-files/ I ran some experiments with the recommended replacement API for NSTemporaryDirectory(). It's not straightforward.

TempDir.swift

        // tl;dr:  FileManager.default.url(for:in:appropriateFor:create:) has unexpected behavior in Simulator and creates temporary folders outside the tmp/ directory.

        let tmpDirClassic = NSTemporaryDirectory()
        print("tmpDirClassic: \(tmpDirClassic)")
        // This is the same code, just syntax sugar for NSTemporaryDirectory()
        // https://github.com/apple/swift-corelibs-foundation/blob/master/Foundation/FileManager.swift#L1420-L1422
        let tmpDirClassicNewShim = FileManager.default.temporaryDirectory
        print("tmpDirClassicNewShim: \(tmpDirClassicNewShim)")

        // Simulator: /Users/steipete/Library/Developer/CoreSimulator/Devices/31C05637-B9C9-482C-A6CE-D063A7CECF64/data/Containers/Data/Application/A68D11A4-88BA-4FCF-A8B1-D102945D0740/tmp/
        // Device: /private/var/mobile/Containers/Data/Application/9D3C6BA4-EE6B-4573-8DC5-66A759D52BC0/tmp/


        // based on https://nshipster.com/temporary-files/ we should use following API, not NSTemporaryDirectory():
        do {
            let temporaryDirectoryURL = try FileManager.default.url(for: .itemReplacementDirectory,
                                                                in: .userDomainMask,
                                                                appropriateFor: URL(fileURLWithPath: "/"),
                                                                create: true)
            print("temporaryDirectoryURL: \(temporaryDirectoryURL)")
        } catch {
            // This prints:
            // Simulator: Unexpected error: Error Domain=NSCocoaErrorDomain Code=513 "You don’t have permission to save the file “Macintosh HD” in the folder “Macintosh HD”." UserInfo={NSURL=file:///, NSUnderlyingError=0x6000008e0870 {Error Domain=NSPOSIXErrorDomain Code=13 "Permission denied"}}.
            // Device: Unexpected error: Error Domain=NSCocoaErrorDomain Code=513 "You don’t have permission to save the file “System@snap-652843” in the folder “System@snap-652843”." UserInfo={NSURL=file:///, NSUnderlyingError=0x280af3a50 {Error Domain=NSPOSIXErrorDomain Code=1 "Operation not permitted"}}.
            print("Unexpected error: \(error).")
        }

        // Using a more appropriate URL works better:
        do {
            // Simulator: /Users/steipete/Library/Developer/CoreSimulator/Devices/31C05637-B9C9-482C-A6CE-D063A7CECF64/data/Containers/Data/Application/CB992FF7-AAC5-4FEF-8839-33122960BB42/Library/Caches
            // Device: /var/mobile/Containers/Data/Application/9D3C6BA4-EE6B-4573-8DC5-66A759D52BC0/Library/Caches/
            let cacheURL = FileManager.default.urls(for: .cachesDirectory, in: .userDomainMask).first!
            let temporaryDirectoryURL = try FileManager.default.url(for: .itemReplacementDirectory,
                                                                    in: .userDomainMask,
                                                                    appropriateFor: cacheURL,
                                                                    create: true)

            // Prints different paths depending on Simulator or device!
            // Simulator: /Users/steipete/Library/Developer/CoreSimulator/Devices/31C05637-B9C9-482C-A6CE-D063A7CECF64/data/Containers/Data/Application/A68D11A4-88BA-4FCF-A8B1-D102945D0740/Library/(A Document Being Saved By URLForDirectoryTest 2)
            // *** NOTICE HOW THIS IS OUTSIDE THE EXPECTED TEMP DIR! ***
            //
            // Device: /private/var/mobile/Containers/Data/Application/9D3C6BA4-EE6B-4573-8DC5-66A759D52BC0/tmp/(A Document Being Saved By URLForDirectoryTest)
            print("temporaryDirectoryURL: \(temporaryDirectoryURL.path)")

            // What happens if we change the appropriateFor URL to be just the App root?
            // /Users/steipete/Library/Developer/CoreSimulator/Devices/31C05637-B9C9-482C-A6CE-D063A7CECF64/data/Containers/Data/Application/CB992FF7-AAC5-4FEF-8839-33122960BB42/
            // /var/mobile/Containers/Data/Application/9D3C6BA4-EE6B-4573-8DC5-66A759D52BC0/
            let appRoot = cacheURL.deletingLastPathComponent().deletingLastPathComponent()
            let temporaryDirectoryURL2 = try FileManager.default.url(for: .itemReplacementDirectory,
                                                                    in: .userDomainMask,
                                                                    appropriateFor: appRoot,
                                                                    create: true)
            print("temporaryDirectoryURL2: \(temporaryDirectoryURL2.path)")
            // Simulator: /Users/steipete/Library/Developer/CoreSimulator/Devices/31C05637-B9C9-482C-A6CE-D063A7CECF64/data/Containers/Data/Application/(A Document Being Saved By URLForDirectoryTest 3)
            // *** NOTICE HOW THIS IS OUTSIDE THE EXPECTED TEMP DIR! ***
            // Device: /private/var/mobile/Containers/Data/Application/9D3C6BA4-EE6B-4573-8DC5-66A759D52BC0/tmp/(A Document Being Saved By URLForDirectoryTest 2)
        } catch {
            print("Unexpected error: \(error).")
        }

Run into a similar problem recently, here is Apple's response: https://developer.apple.com/forums/thread/735726?answerId=762035022#762035022 basically what they are saying is that itemReplacementDirectory should only be used, literally, for item replacement, which means copy the temp file to this directory, edit the file, then move it to overwrite or save as a new file.