Thank you for your reply MrSteveVee it's really appreciated.
Although in this moment I'm not, yes, normally use an Adobe RGB display.
Thanks to your message I have been doing more testing, and, contrary to what I was thinking, it this does not seem to be Adobe RGB related...
I made a test with RAW images and I'm having the same issue.
TESTING STEP 1
In the left side you can see the Darktable preview, and in the right side the exported PNG file
TESTING STEP 2
I tried to close DT and to rename the Darktable config directory
/home/my-user/.config/darktable so reopening DT a new flesh config dir has been created.
The interest thing is that now the working profile assigned by default is not linear2020RGB default, but sRGB
As I can see the two images colors are pretty consistent.
If this was the problem it was probably due to the DT update I have made in the past.
I'll do more testing in the next days, so if there will be any news I'll post them here.