Yeah, I experienced the same "problem" several months ago. I reached out to TAC as well as the Developers about it. This appears to be a feature limitation. According to the Parameter Reference Guide for Cisco Jabber 11.5;
http://www.cisco.com/c/en/us/td/docs/voice_ip_comm/jabber/11_5/CJAB_BK_J06BEF2D_00_jabber-parameters-reference-guide-115/CJAB_BK_J06BEF2D_00_jabber-parameters-reference-guide-115_chapter_010.html
30 (default) is the MaxNumberOfBookmarks
I am a little surprise you were able to join 32 chat rooms since TAC conducted a test and claimed, they were only able to join 30 chat rooms after they changed the MaxNumberOfBookmarks to 35. When they joined the 31 chat room, they received an error message which mirrors my results as well. By any chance, are you accidentally counting P2P or Adhoc Chat Rooms?