Was one of those clothing pieces you mentioned managed by
Stopper's Clothing Plugin Manager by any chance?
Bit of a story time here.
Recently I've also ran into this issue, thought it was really strange because it's an issue I've never encountered with Timeline before. I'd open up Timeline and certain buttons in the Pose tab such as "overwrite pose" and "copy pose" suddenly don't work and I'm no longer able to save the starting pose. Initially I thought maybe something was up with my Timeline... even though the issue only started happening recently despite me not changing my Timeline version AND the issue now seemed to be occuring on Timeline plugins on scenes not only made by me, but others as well. New random issue just seemingly outta nowhere.
At first when I read your comment I was like... "clothing? why would clothing affect Timeline in that way?" But I decided to test it anyways. I loaded up a new Female Atom, added Timeline, messed with the Pose tab a bit, and everything seems to be working just fine. However, after loading a clothing preset I've been using a lot recently, those certain buttons on the Pose tab like "overwrite pose" and "copy pose" stopped working again. Turns out the culprit was that one of the pieces of clothing in my clothing preset was using Stopper's Clothing Plugin Manager.
View attachment 428313
Once added it completely bricks the pose functionality of Timeline and overwriting poses is no longer possible on that Atom. Removing the clothing item doesn't fix it, neither does removing and re-adding the Timeline Plugin on that Atom. The only solution I'd found is to remove the clothing item from the Atom -> save scene -> restart VaM and reload scene and it's back to normal. I also tried it with some other clothing that uses Stopper's Clothing Plugin like this one below and had the same result.
View attachment 428315
TLDR: Currently, adding clothing that uses
Stopper's Clothing Plugin Manager seems to bug the pose functionality on Timeline
I'd be curious to see if anyone else would be able to test this and get similar results. If it's truly the cause, It's something we outta let Stopper know.