I've e-mailed this issue/feedback and linked this forum topic via https://www.nokia.com/en_gb/phones/support/
So far HMD/Nokia has shown no indication that it is prepared to implement it's own core functional improvements and enhancements to it's phones.
Well they got back to me and said they will pass these suggestions on to the devs, so fingers crossed!
Well.. since official 8.1 Oreo the issue didn't occur anymore and i'm running it for over a week now.
So it seems to be fixed.
I am having the same issue with Nokia 6. It is annoying this much, that I want to disable multiwindow, but there is no option for this. Nokia please improve this part of you product.