Ah! I just found out something else: the effect seems to be connected with camera settings. The T5 has a crop mode calls “sport mode”. The black bands on the side appear also with other lenses when this mode is turned on. Good to know - seems to be a bug anyway.
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If you feel its a bug, please report it on github.
The sports finder crop mode has never worked well for me in Darktable, regardless of camera model. A small crop does the trick, though.
I did. The topic itself seemed to be well known.
“regardless of camera model” - thats interesting and makes me understand the comments of the developers I read yesterday. I will continue cropping 
Would it be possible to define an auto-applied preset for that situation (i.e. is there something accessible in the metadata that identifies that mode)? Of course, manually applying a preset is always an option.
IIRC, I did build just that into my automatic fuji styles. But you need to read the metadata with exiftool, as they’re not available within darktable itself.