Thanks Harley,
Will give it a go. Already have VS2019 installed, and I've used Subversion in the past (and even C and make in earlier Unix days!) Just need to clear out a few hundred GB of Mantaflow cache files to make room first ;-)

Thanks Nathan, I really appreciate your generous reply! I was feeling a bit of a fool for reporting it as a bug, so you've made me feel a lot better about it. I'm sure I'll get used to the behaviour however it works out. I'm a retired developer and still dabble with Visual Studio, .NET and C#, so perhaps I'll get more involved with the "other side" of Blender someday :-)

Thanks Germano, apologies for not seeing this in the release notes.

I have to say I'm not a big fan of this change. I'm relatively new to Blender (after a long gap from pre-Cycles days), so apologies if I'm talking nonsense here, but ...

I've discovered some additional behaviour that may help:

I've realised that 2.91 also has the new Display Filter but doesn't show this problem, so it looks like it's not a direct result of the Display Filter change.

Thanks Robert.
To clarify my second suggested fix, if the Infotip continued to used the cumulative time, but divided it by the total rendered tiles (i.e. completed frames x tiles-per-frame, plus completed tiles in current frame) then multiplied it by the total remaining tiles (i.e. remaining tiles in this frame, plus remaining frames x tiles-per-frame), then the calculation would work correctly for both a single render and for an animation, and would not require any special handling. For consistency the progress bar would also show this overall progress %, rather than just the current frame's progress.