I notice the following:
create a 4/4 signature on bar 94
create a 3/4 time sig. on bar 93 > it gets created on bar 92 !!
The other way around works. See gif
Is Insert mode active?
no it is not active.
Side note: is there a counterpart of INSERT? It seems I can delete but the following notes stay on place
This is happening because Dorico won’t delete beats when changing a time signature. If you reduce the bar to 3/4, the extra beats have to go somewhere before the next time change, so it creates an additional bar to put them in.
Yes, to be clear: this is not a bug.
see also
you get used to it after a few times 