I found a small mistake in the RO_RSB_PSLV config file.
The PSLV PSOM (XL) part (line 932) is lacking the following parameter:
"!RESOURCE,*{}"
Because of that, that srb in game has the solid fuel resource as well as HTPB. Using that line that i copied from the non-XL PSLV Booster, the solid fuel disappears and the part performs as it should.
Also, there is a typo in the PSLV interstage adapter title (lines 341 and 343). It says PLSV instead of PSLV.