I noticed that.. 4 months ago, according to gitlab. I'll just paste what I filed there:
pd->graphics->newBitmap() is rounding rowbytes up to 32-bit boundary
We started allowing 8-bit alignment a while back so that fonts are encoded more efficiently. Before we change it here too we should check for performance differences. I doubt it'll be anything noticeable but 32-bit aligned access is slightly faster; on the other hand, more padding increases cache misses.
So yeah, it was just an oversight, and left at low priority because we hadn't heard of anyone having any problems with it. I've attached your report to the issue and bumped priority up a notch, fyi. Thanks for the note!