Hello,
not really a bug; when you start rendering, the noise pattern is randomized, so if you have smaller increments (3x10 passes), this will add up eventually and take a bit longer to clear up. Also it is possible that adaptivity does not kick in immediately. If you increase the increment, you will gradually get better results faster (peaking at 1x30, obviously).
We may look into whether the adaptivity can be improved for Resume last as we have all the data necessary for it to kick in, but that's about it.
Thanks,
Jan