Now writing from the computer...
@Splinty --> I have 'fought' a bit with my phone and managed to install English as a language for the keyboard. I should have done that initially.
I am using an Android phone, and if I either set the keyboard straight to english, or "allow English word suggestions for the autocorrect", then most of the problems disappear.
One bug that I can still provoke is, if I write a word, like "problem", then click the cursor on the phone right in front of the first letter (so, between " and the letter p from the word problem), and try to write, e.g. "problem" again. So I would have "problemproblem" and then press enter, it gets messed up and writes "problemproblemem". This is not a fun game, but happens, say, when I forget a word and want to squeeze it in in between a sentence, then the phone always messes up words. Before I had installed English, it would erase words and leave just the first letter every time. That had happened also when I pressed space. Now, with the english setting it just occurs when pressing enter (erasing the second word happens still on the german keyboard with the english autocorrect suggestions though).
Another one, if I have my "problemproblem" freakshow word and put the cursor on the phone somewhere in between those letters (on the phone, the whole word gets underlined so show that it recognizes it as a word of sorts) and press space, it erases everything that is to the right side of the cursor. That is especially annoying when you want to separate two words and the site erases the second word. big fun.
The original sentence that gave me problems in my original post, where I was trying to write "I have to proofread and edit shit" only works with the english keyboard, otherwise it messes me up every time. With different effects and errors mind you. It's also not because of the word shit. It does the same with "I have to proofread and edit stuff". So either it's the chinese government playing a silly joke on me from my Huawei phone, or that sentence might be a good hint as to what could cause that bug.
--> Anyway, if I set my keyboard to English, most of the troubles can be avoided. Only when two words get glued together and I want to separate them I still get the problem that the second half of the word might get erased.
Thanks for checking!