Hello John
Not directly connected to this issue: the keyboard layouts have never been a strong point of macOS, even if, beginning with 10.2, it has been quite easy to add custom ones. I could not test all the keylayouts, but—as a mere example—the Extended U.S. keylayout (now renamed ABC Extended) has impossible non-mnemonic Option combinations. I solved them with my US Academic. To say nothing of the basic U.S. keylayout, with its eerie chars at the Option level. This is entirely bizarre, because the initial solution was very good, i.e. the basic Option combinations for acute, grave, umlaut, cedilla and circumflex.
For many years, users of Hungarian and Romanian keylayouts could not access one essential char if using a U.S. physical keyboard, only with the European ISO extended keyboards. Meanwhile, there is a solution for Romanian, I do not know whether for Hungarian as well (have not tested this, but it was a discussion on this forum several years ago). This is why that banal extra key left to Z is very useful in Europe. Nevertheless, many people buy macs in U.S. or elsewhere with the U.S. physical keyboard. The discussion may be longer, but I stop here.
In this context, I do not wonder that the Ukrainian keylayouts may be problematic, it would not be the only case with Apple keylayouts. Of course, I repeated sent them feedbacks on this issue, then I stopped: it was entirely useless, they do not care. Nevertheless, in this very case, I suspect he did something illegal with the system, I cannot figure out what, so the reported problem is not exactly an Apple issue.
Anyway: I think macOS has a (much) better solution for the custom keylayouts than Windows, to say nothing of Linux, at least from the simple reason that UKELELE may add new functionalities. Also, macOS allows a simple solution to add these keylayouts to the basic set.
Unfortunately, things are a lot worse (Windows or Linux style) with iPad OS, in fact iOS running on a larger screen.
Best from Bucharest