"Fillet" separates the whole object. My workaround is as follows: Create Fillet, then copy and Ctrl + Z.
Now reinsert and with the outer edges of the "filet" the entire object trimming.
Then together with "Boole Union" the fillets with the object.

This is very awkward, but at the moment seems not to be working differently.
If you want to try, may gladly do it.
You can see the critical area in my screenshot.

I used the *. Rar format, since it is much smaller.


Regards
Jörg