Paste All from Excel clipboard has inconsistent errors

[Bumbles]

Hi everyone.
Win 10, Office365 subscription.

For some time now (I'm not sure exactly but perhaps a year or more), I've had an annoying and random problem with the clipboard.
I can copy several items onto the clipboard (nowhere near the limit of 24) and sometimes I can "Paste All" and other times it only pastes some of the items.

For example, I typed numbers 1-9 in cells A1-A9.
I copied each of them individually (makes no difference if I use right-click>copy or Ctrl+C)
They each show up in the clipboard pane.
I then click on Paste All, and sometimes they will all paste OK but most times only a random selection of them will successfully paste.
Screenshot shows ten attempts at clicking Paste All... you'll see what's happening.

I've tried searching periodically online for the cause but never been able to find one.
I have tried unchecking "Live Preview" in the Options > General.
I have tried running Microsoft's Office Repair tool.
There are no macros/VBA running and none of these workbooks even have macros enabled.
I first noticed it in a workbook I use frequently, but it happens in other books too and even in brand new "Book1"s. (As is the case in the attached screenshot)

It even happens if I try using the clipboard in Word, from Excel
Screenshot also attached showing four attempts at pasting all in Word.

In case it's relevant, I've noticed that sometimes the pasted area remains selected (grey), and other times Excel will paste whatever it pastes and then move the active cell down to the next cell below the paste area.
There doesn't seem to be any connection between that quirk and the success or failure of paste all.

Any suggestions?

 

[Bumbles]

I don't think I can edit my post above, so am adding a detail here.

I just repeated the same steps on my wife's laptop. She also has Excel, as another user on the same account (we can have up to five users).
Her Excel installation does exactly the same thing.
Sometimes Paste All works with the steps above, and sometimes it doesn't.

It kind of makes the whole idea of using that clipboard to "Paste All" useless. Even potentially dangerous if you just trust that it has pasted all your data and continue merrily working along without noticing.