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Finding that the 'REDO' function (as in UNDO and REDO) sometimes does not work as I expect. Here is the work path:

- Start with version 1 of a drawing.
- Erase something.
- Redraw it. Now it's version 2.
- Play it. Want to compare it to how it looked in the first place (version 1), so...
- Hit UNDO to get back to original version 1.
- Play it again. Actually prefer the fix (version 2), so...
- Hit REDO in order to go back to the redrawn version 2.
- REDO beeps and does not work.

Same issue editing on the timeline:

- Deleted an instance.
- Made an adjustment to timing of another instance.
- Hit UNDO to get it back as it was.
- Prefer the change, so want to REDO the above action to delete instance and adjust timing, but hitting REDO beeps and does nothing.

Is this a bug or the way it is meant to not REDO? Having said that, sometimes it seems to REDO just fine, so I'm confused.

The easiest way is to make use of the "History Panel". I keep mine always opened and close to the project panel. When you are ready to experiment to find which way you prefer, check the box to the left of the top most line item. Now continue animating and so on.
When you are ready to try a different way, click the box to the left again. Now look at the three arrow symbols at the top right. Click the left one which will return you to exactly the previous mark, which is where you started your animation. Using these markers you can easily navigate from place to place in time.

Okay, this is going to give away that I am new at this. I wasn't aware of this History panel, which is great. I use it in Photoshop all the time. If nothing else, it may illustrate why the REDO is not working, but very handy generally. I have to avail myself of all the panels in that menu.

Thank you Paul (again!).

yep, actually, the historic panel is the way to go.

for your purpose, checking last / previous version of a wip drawing, I'm sometimes still using
this (pre-historic) alternative, invoking the Spare page :

- version 1 of the drawing,
- Copy-to-Spare ( hitting Ctrl[J] shortkey for me, it could be a different one for someone else)
- re-work, ... up to version 2,
- swap to version 1 / version 2 / version 1 / version 2, ... by hitting the [J] shortkey (it could be a different one)
(here, [J] is for "Spare Swap", or "Spare Exchange", 2 appelations for the same thing),
when happy with the version 2, but feeling it's still improvable, I "copy-to-spare" it by hitting Ctrl[J],
- continue to work on it ... up to version 3,
- swap to version 2 / version 3 / version 2 / version 3, ... by hitting the [J] shortkey,
... and so on, until I'm plenty satisfied, then go on to the next drawing.
basically, it consists to keep the version n-1 always at hand (on the spare page), ready to be recalled,
just in case, ... you know, repentance is a common feeling for artists.

ps : I personnally use a custom "Copy-to-Spare" command, assignated to my Crtl [J] shortkey,
which Kills the existing spare first, then copy (the current frame) to the spare page.
with this one, I skip the perpetual popup question " ... blabla ... do you want to replace it ? ",
question which requires an answer "Yes" or "No" to go on (breaking my workflow),
process that could become tedious when repeatedly I have to Copy-to-Spare on a not empty spare page.

ps2 : the advantage remaining of this "pre-historic" working way, is that you can make easily a sort of "mix" of the 2 versions,
by restauring part of the (spare) version n-1 by drawing (filled stroke tool) on the version n in Merge mode.

Another way, of course is to simply have your backups memory set to a decent number and you can always depend on the availability of retrieving earlier results of your work and bringing back any of these backtracked layers to your current clip.

And yet another way (which I use often) is to go into the Project tab and make a copy of the current clip, which is an easy and fast thing to do without even having to rename anything.

Notwithstanding the helpful suggested workarounds, still finding the issue with the Redo command frustrating. I think I will report it as a bug.