Gist demonstrating Publishers.CombineLatest is racy

CombineLatestRace.swift

func testCombineLatest() {
    for _ in 1...100 {
        let e = expectation(description: "Each subject should emit true")
        let s1 = CurrentValueSubject<Bool, Never>(false)
        let s2 = CurrentValueSubject<Bool, Never>(false)
        let cancellable = Publishers.CombineLatest(s1, s2)
            .sink {
                if $0, $1 {
                    e.fulfill()
                }
            }
        for subject in [s1, s2] {
            DispatchQueue.global().asyncAfter(deadline: .now() + .microseconds(10)) {
                subject.send(true)
            }
        }
        wait(for: [e], timeout: 5)
        cancellable.cancel()
    }
}

You have to use the same serial queue for all Publishers that you would like to combine. After combining, you can dispatch the sink on any queue, concurrent or not:

class testcombineTests: XCTestCase {
    
    let queue = DispatchQueue(label: "MyQueue") // serial queue

    func testCombineLatest() {
        for _ in 1...100 {
            let e = expectation(description: "Each subject should emit true")
            let s1 = CurrentValueSubject<Bool, Never>(false)
            let s2 = CurrentValueSubject<Bool, Never>(false)
            let cancellable = Publishers.CombineLatest(s1.receive(on: queue), s2.receive(on: queue))
                .sink {
                    print("\(s1.value), \(s2.value)")
                    if $0, $1 {
                        e.fulfill()
                    }
                }
            for subject in [s1, s2] {
                DispatchQueue.global().asyncAfter(deadline: .now() + .microseconds(10)) {
                    subject.send(true)
                }
            }
            wait(for: [e], timeout: 15)
            cancellable.cancel()
        }
    }

}