I use the desktop site on mobile. To be precise, I use chrome browser in One UI 4.0. However, when I copy and paste, the pasted content doesn't seem to be recognized. (If the Add button is not activated and try to enter additional text, only the entered text is displayed.) The 2017 wikitext editor seems to have the same symptoms. Sorry for bad english.
Topic on Talk:Talk pages project/New topic/Flow
Are you trying to copy/paste formatted text (such as links)? Does this happen even if you copy/paste a single word of plain text?
(Your English is good. If you would rather write in another language, that's okay. MediaWiki.org is a multi-lingual wiki.)
Perhaps the symptom only appears when using some wikitext (eg *, {{}}). Copying plain text doesn't cause that symptom.