Twitter_Uncircle.user.js

// ==UserScript==
// @name         Twitter un-circle
// @namespace    https://gist.githubusercontent.com/DesertEagleDerek/afa3281b951b4b05c27422aa2ff104ee/raw/Twitter_Uncircle.user.js
// @updateURL    https://gist.githubusercontent.com/DesertEagleDerek/afa3281b951b4b05c27422aa2ff104ee/raw/Twitter_Uncircle.user.js
// @downloadURL  https://gist.githubusercontent.com/DesertEagleDerek/afa3281b951b4b05c27422aa2ff104ee/raw/Twitter_Uncircle.user.js
// @version      0.9
// @author       DesertEagleDerek
// @description  Revert to older square elements
// @homepage     http://www.deserteaglederek.com/
// @match        https://twitter.com/*
// @run-at       document-start
// @grant        none
// ==/UserScript==

function addGlobalStyle(css) {
    var head, style;
    head = document.getElementsByTagName('head')[0];
    if (!head) { return; }
    style = document.createElement('style');
    style.type = 'text/css';
    style.innerHTML = css;
    head.appendChild(style);
}

function addStyleCSS(values) {
    var finalCSS = "";
    for(var i = 0; i < values.length; i++) {
        for(var j = 0; j < values[i].length; j++) {
            if(j > 0) {
                finalCSS += values[i][j] + " !important;";
            } else {
                finalCSS += values[i][j] + " { ";
            }
        }
        finalCSS += " } \n";
    }
    addGlobalStyle(finalCSS);
}

(function() {
    'use strict';
    // array
    var cssArray = [];
    // get rid of those circles! (pre-2019 layout)
    cssArray.push([".Avatar, .ProfileAvatar-image", "border-radius:8px"]);
    cssArray.push([".ProfileAvatar", "border-radius:12px"]);
    cssArray.push([".ProfileCard-avatarLink", "border-radius:6px"]);
    cssArray.push([".ProfileCard-avatarLink img", "border-radius:5px"]);
    cssArray.push([".DashboardProfileCard-avatarImage", "border-radius:5px"]);
    cssArray.push([".avatar", "border-radius:5px"]);
    cssArray.push([".EdgeButton", "border-radius:5px"]);
    cssArray.push(["#tweet-box-global", "padding:5px"]);
    cssArray.push([".RichEditor", "border-radius:5px"]);
    cssArray.push([".tweet-content", "margin-bottom:16px"]);
    cssArray.push([".dropdown-toggle", "border-radius:8px"]);
    cssArray.push([".dropdown-toggle:before", "border-radius:8px"]);
    cssArray.push([".Gallery-media", "border-radius:6px"]);
    cssArray.push([".ProfileCardMini-avatarImage", "border-radius:4px"]);
    cssArray.push([".DMAvatar", "border-radius:4px"]);
    cssArray.push([".avatar--circular", "border-radius:4px"]);
    // get rid of those circles! (2019 layout)
    cssArray.push([".r-sdzlij", "border-radius:4px"]);
    // add style to page
    addStyleCSS(cssArray);
})();

I've experienced this a few times but not recently. I use Google Chrome and more often use TweetDeck rather than Twitter itself and it never seems to happen on there. Whenever I witnessed this behaviour, it seemed to be when the site was taking too long to load. One thing you can try is in the header section of the script, change the run-at from document-start to one of the other accepted values. You can find a description of the values at the link below.

https://www.tampermonkey.net/documentation.php#_run_at

Aha! "Document Body" seems to have fixed it for now. My "stress test" is to open 10+ new tabs (by, say, mass opening people's profiles from my geed) and then checking once loaded to see whether the elements are circles or squares. I will keep testing and see whether that remains the case. Thanks a lot!