I can't post in forum when using google chrome & mozilla firefox in android phone unless I set them to 'desktop site' mode.
Even then, I cannot perform 'tap and hold' to brings the copy-paste panel in the text field. The panel disappear in split of second.
I'm using android 5.1.
The phone I use is not Android, but I have noticed some odd stuff that happens with copy/paste or the “source” button (or maybe it was a combination of the two). It is almost the opposite of what happened to you: it copy/pasted stuff I never copied. And I couldn’t see the error until after I had sent it. It also puts random question marks everywhere, but those are minor weirdities.
I don’t know if this just happens to me or if others experience anything like this, but I hope you find a solution to your copy/paste issue.