I was able to riff off this to get around this problem in web client (outlook.live.com). I had this issue under Win10 on multiple browsers, but only with a long running email thread from one sender. This mail account has never been attached to a device, I've only ever used it via web client.
Open the email you want to use
Click the [...] button next to reply/replyall/forward
Choose View>Open in new window.
In the new window click reply/reply all
You should already be able to tell its working, because the full mail thread history will appear in the reply body
Send your reply as normal.
After this the mail history/reply issues for that recipient are gone.
This has to be a service issue and its annoying that I can't find any official acknowledgement of the problem. I see a lot of talk about Cache rebuilds on the Office support twitter and expect this is collateral damage.
I signed up specifically to give this feedback, I hope it helps someone.
https://docs.microsoft.com/en-us/answers/questions/386243/outlook-asking-for-34changekey34-upons-clicking-34.html