See also update below.
For some time, I have been using Unicode Hex Input 99% of the time. Recently, with some assistance here and from the author of Ukelele, I managed to add press-and-hold pop-ups to Unicode Hex Input, which eliminated any need for any other IM or keyboard for European languages. But a few days ago, possibly with the upgrade to 10.15.4, MacOS has taken to switching me to Apple's default (U.S.) every time I change focus from one app to another or from one Safari tab to another.
Is it an Apple bug, or is there something new I missed in the release notes? Is there a simple fix/workaround?
Something similar happened in 2016, but that fixed itself without my doing anything (as far as I was aware).
UPDATE 1: Furthermore, my list no longer has an input method for Chinese, and cannot find either "Chinese" nor "Mandarin" on the list of languages. The press-and-hold pop-ups no longer work, and the keyboard viewer & character palette are also no longer available (show keyboard & emoji viewers in menu bar is unchecked and turning it on doesn't get saved).
UPDATE 2: Following Tom's suggestion, installing the 10.15.4 Catalina combo restored the availability of Chinese, but removed all input methods but Turkish and Greek, which prevented typing a login password! Had to select "other" and reinstall them on the login screen, but had to do it twice before it would stick. And then trying to do press-and-hold would disable the keyboard completely. I could restore it, though, by changing keyboards. Hold any key down too long, even one with no press-and-hold options defined, would make all keys no longer functional until a different keyboard is selected with the mouse.
UPDATE 3: Today (9 April 2020) Apple released a minor update (did not change version number) which fixed both press-and-hold problems I described in update 2!