That does indeed feels like a potential bug, though I've never seen that happen before. I'm actually somewhat confused by exactly how that happened. Does Ctrl-R do the same thing? If you need, you can disable aspect ratio lock by holding down shift while resizing.
Yes, Ctrl+R does the same thing and I never knew about the shift thing, pretty cool. Though idk, I'm super OCD with like my face not looking like it should due to resizing it without the aspect ratio locked (like it would be too narrow or too wide, i want people watching me to see what i see. idk im weird)
But anyway, I really wonder what causes this. Seems to only happen when I try to resize it with the aspect ratio lock on. Only appears to happen with that particular thing too, doesn't happen with my desktop capture source or anything pictures. Maybe it's only a Video Capture Device issue? I would try it with a Dxtory source but I am lazy atm...