I don't think that's a bug. Looks more like numerical artifacts caused by SPICE trying to optimize for a minimum number of steps to compute, yet the duration is very long (20 seconds) and the digital circuit is millions times faster when compared with those 20 seconds.
Yes, it's exactly that. If you zoom in on the "oscillations", you can see that those are jumps of discrete values (it looks like a 'triangle' wave). This is typical of an integration error, which comes from the fact that there are very sharp edges relative to the total simulation time. Spice will try adjusting the simulation steps automatically but there are limits to this.
Specify a maximum allowed timestep (for example 1 microseconds). Use ".tran 0 20 0 1us uic" instead of ".tran 0 20 0 uic", and the ringing at each edge will appear reasonably OK.
Yes, this is how you can work around this.
Note that I'm a bit surprised here, because I would have expected the logic primitives in LTSpice to be pure digital simulation - LTSpice is a mixed signal simulator - so that I wouldn't have expected integration errors on a digital primitive output, but I don't know how it's really implemented behind the scenes. Possibly it's the "analog-to-digital" interface of the primitive that causes this.