"I haven't enabled another language than English" definitely doesn't mean "I only edit the English Wikipedia". On en.wiki, click on the gear next to "Languages" in the left sidebar. Those are the Universal_Language_Selector settings. Both display and input ones should be pointing to English (actually the input one should be disabled, I think).
This said, on the English Wikipedia, this is your only edit with the visual editor ever. So, one idea is that you could mean that the problem is about the 2017 wikitext editor instead, as that's the one you seem to be using regularly (although this may or may not make a difference). To see whether this changes anything, you could go into your Beta preferences at en.wp, unselect both "Automatically enable all new beta features" and "New wikitext mode", save, and see if the shifting issue also occurs with the older wikitext editor you are using now.
There are several other things that we may suggest if this doesn't solve. However, it requires a bit of patience and collaboration. For example, you could have suggested a single article where the issue always happens, state at which point specifically you started editing, etc. You could also have tried in a sandbox and saved the final result. The amount of details may look irrelevant to you, but it's what helps others figure out what is happening. (Just looking at your previous contributions there doesn't help much, sadly.)