Rare issue I have with making symbols in Animate

[RedFields]

I just want to know what possibly happened that causes this, because this is genuinely a nuisance.

Ill copy some keyframes into a symbol to make a repeating animation, but very specific frames (like in the screenshot) even when I copy and paste the frame again, just decide to lose a portion of the drawing and refuse to let me paste the frame unless it removes certain portions. 

In this case it was easy to just redraw or manipulate other keyframes to make it look the same, but there have been times when half of the pasted keyframes are just missing outlines and fills and refuses to let me paste.

[Not an alien]

I've never run into this specific problem, although I don't think I understand exactly what is happening.  Are you copying actual frames from the timeline, or are you copying the shapes in those frames? It looks like you have a range of frames activated on the timeline too. Try disabling the multiframe selection. 

I would make sure that you have unlocked the frames you are copying from and into.  Not sure this helps, but now you also have the option of converting a layer to a movieclip. A convenient way of copying / moving parts of an animation.