Hello. I could not find this bug when searching, but it's probably somewhere in the sea of bugs.

This bug is caused by the /fill command (see command below). When pasted in a superflat world inside a chunk it is supposed to create a 16x16x16 chunk of oak fences. However when executing this command in 1.15.1 or 1.14.4 the command generates an area of 17x17x17, which is one more column/layer in the 3 dimensions. I have made a video to easily show the bug which you can find below. I also showed another bug in the video which already has been submitted, so ignore that one!

Command used: /fill ~0 ~0 ~0 ~16 ~16 ~16 oak_fence

YouTube video: Minecraft - Fill command bug. 1.15.1 & 1.14.4