This isn’t really a bug, but perhaps an oversight.
In R3b3 the detached/attached state of the sampler window is saved and restored on next start-up. That’s a good thing imo, but if the detached sampler window has been closed, Renoise should start with it closed next time (but in detached mode). That would feel more consistent I think, and the sampler window is a bit obtrusive on start-up if you don’t really expect it but still prefer running it detached.