Diamonds deleted? That doesn't sound right, unless you also got your credits back.Ctrl+C works on Windows, but you may have to select the text first. Right click doesn't work and is not planned atm.
80-90 diams gone - I thought you had reset everything. TBF I'd have no problem with it - restarting from scratch isn't bad while testing.
Since you argue it wasn't intentional and I haven't deleted itch.io cookies, idk what happened.
Could you describe the steps? If you rebirthed you should see an apparent reset, but keep diamonds plus everything you bought using diamonds. I cannot reproduce, tried a few times.
I appreciate your proposal - still: no worries (and last night was... busy ^^ so whatever happened, part of it won't be recovered).
I wish I could describe the bug in details. I can't, so won't.