about the time I upgraded to TeXmacs 1.99.16, I noticed that the `IV' button (used to add equation numbers) stops working after some time TeXmacs has been used. The menu item Focus > Numbered, however, continues to work normally.

Restarting TeXmacs fixes the problem. Upgrading to 1.99.17 and deleting the .TeXmacs folder did not help, i.e., the bug comes back after some time. I am using the macOS version.

This bug has been observed by Massimiliano Gubellini as well ('I confirm that there is something fishy with that button') and by -email is unavailable- ('I find another case that the focus bar button rarely works: when you edit a beamer and you form a unroll list (or enumerate / description), and later you want to change it to unroll greyed. I find that in most cases, I usually failed to do it by clicking the button in the focus bar (to be more precise, it is in the configure submenu after clicking that button in the focus bar which looks like a wheel). It is weird: you can select that "greyed" option, just no effect. However, I can do that by choosing the similar option in the Focus menu.')