In my Minecraft survival world on the version 1.16.4 me and my friend noticed, that a cactus in a pot becomes partially invisible after an upside down slab was placed above it. If the slab is removed, the cactus returns to being a normal looking cactus in a pot.
Duplicate of MC-195461, which was fixed in 20w46a. You are experiencing this issue because you are playing on 1.16.4.