I would qualify it as an unintended side-effect of intended systems (aka a bug)I don't know if this qualifies as a bug.
Gonna go with no, since clustering join/leave messages is presumably intended to reduce visual clutter when no messages are being sent- is this desirable (probably not) or undesirable (doesn't personally bother me) and
Gonna go with no, but no harm having it identified in case someone scrolls through this forum on a bad day- is this worth dev time to fix/implement (right now? probably not unless someone else wants to take a crack at it)