I’m seeing this on Windows 10, Anki version 2.1.54 (b6a7760c), Python 3.9.7 Qt 5.15.2 PyQt 5.15.5
If I’m editing an equation in Anki in the new live preview, if I alt-tab away to focus on another window and then alt-tab back to anki, the live preview is lost, and the textbox itself does not have focus anymore. This means to get back to typing I have to either click again on the textbox/equation, or shift-tab to focus again on the textbox, and then navigate with the arrows back to the equation.
Contrast this with what happens if we’re just editing normal text. Going to another window and then coming back returns you with the cursor in the original position, so one can resume typing without further delays.