Not a major issue and unclear if it is a bug, but nonetheless a bit annoying.
Seems that when Renoise is setup to clock an external MIDI input (Cirklon) and you press play in RN without starting MIDI clock, the timing goes all weird until it kinda stabilises after a bar or two. When not receiving MIDI clock, it would be nice if it just ignored the MIDI clock setting and reverted to normal internal clock behaviour.