uMap - version restoration does not work

I spent an afternoon adding objects to four layers of my first uMap project, saving after every added object. As I revisited the map a bit later, all but one object I added to one of the four layers completely disappeared. It was not the last layer I added objects to either, so I am assuming it was not a problem of saves being cut off after a certain point. The other layers still show every object I added to them.

So I went to check out the layer version history, and as far as I can tell, the objects are still included in previous versions of said layer:

The saves up to the 30.61Kb one are consistent with my gradual adding of objects, as is the sudden disappearance of almost all of them after that point.

Now what gives me a headache is that I cannot restore any of the previous versions. The objects still won't show, regardless of which version I choose.

Can anyone tell me if I missed something here? What could have lead to this issue? Is there a solution or at least a workaround to this problem so I can salvage the data? Of course I could go back and add the objects again even if take me several hours, but I'm wary of this happening again unless I really understand what happened - plus, the data still seems to be around somewhere, so I would be glad to find a way to retrieve it. I'm grateful for any and all hints and support!

(Apologies if this question ends up being asked twice - my first attempt seems to not have gone through.)