Hi sorry I missed your note before. And also no I didn’t see that. To be honest I find this particular forum software confusing to navigate. I mostly will just search at the top and click links like the one you were kind enough to provide. 
So what you are getting at is this, ya?
I did reset everything everything to as basic as I could make it. Did not reapply the config backups. Did not keep settings
It seems to me that the source of this (if it is indeed not expected of this model/firmware combo) could be discovered by comparing the files from a brand new out of the box unit and then what happens when you perform various tasks. Here is a thread I found that have info on how this could be accomplished: directory - rsync compare directories? - Unix & Linux Stack Exchange. Or a very rudimentary way maybe by sudo tree -aif --du / > output.txt or something less silly.
The thing that is required is an image or copy or smthg of the entire storage of a fresh device.
I can’t do this because my router is now contaminated and also I do not understand this filesystem at all. But maybe someone with a bit of know how and resources would figure it out.