I’m afraid I have seen something similar with iOS 13. I have several languages for both keyboards and in my preferred language order. My native language is English which is my preferred and I write most of my texts in English using the English keyboard. However, every once a while I’ll write a text to an old colleague in their language. I switch the keyboard over and sure enough word suggestions make sense.
Then came iOS 13 and I started noticing how in the middle of an English text to my wife using the English keyboard, I’d get suggestions for Italian words. Now my wife doesn’t speak Italian (not that mine is really any good) and hence I’ve never used the Italian keyboard to text her. But I have used the Italian keyboard to text an old buddy in Frascati in my broken Italian.
I haven’t been able to find a setting to turn off these nonsense suggestions. My best guess is Apple changed iOS 13 behavior to be “smart” about what to suggest based on what other languages or keyboards people have selected regardless of what keyboard they’re presently actually using. Now in some instances that makes sense because multilingual people will often mix languages when they communicate. So a German/English speaking couple might use an English keyboard to exchange texts in English but still every once in a while use a German word (especially when there’s no English equivalent, such as the beautiful sternhagelvoll). In those cases Apple’s new “smart” approach (assuming this is actually what’s happening here) would make a lot of sense.
On macOS after all they’v already done something very similar for a long time. We can have multilingual spellchecking where macOS will recognize from the context which language you’re in (I’ve checked this works down to a quite granular level) and apply that spellchecker to only that part of your text.
The problem with such “smart” support is usually just that. It’s not quite as smart as we are. Which is why it would be nice to have a setting where you can turn that off on iOS if you want to force it to suggest according to the language you’re typing in. For people that never or only rarely mix, that would likely require far less manual correction, while for those that mix a lot the setting would allow them to have iOS mix suggestions dynamically matching their use.