At this point I wouldn't worry about doing my test. The results you've gotten, Rharv's testing and mine indicate to me there's no problem with a 255 bar limit. Importing XML files is a fairly new feature with BIAB and RB and I have no experience or knowledge working with them, within or outside of BIAB/RB.

I think there's a command from the XML file that is either being misinterpreted by RB or something along those lines. There are several here on the forum that are familiar with XML files that can help you sort that out and you can clear up this issue with little trouble and begin to enjoy your music.
BiaB Ultra Pak+ 2020:RB 2020 Build 1. Dell Inspiron AIO Desktop; Windows-10-64 bit, 7th Generation AMD A9-9425 Processor Five Core 3100 Mhz and 16GB DDR4 Memory.