Well, in this case a video is better than explaining, but.... Take a look at what the block is doing:

https://clips.twitch.tv/jonbams/PoorWeaselArsonNoSexy

It moves by itself :S

Do you know if he is playing in single or multiplayer?

Yes that happens on a Single Player World.

Yeah, thats not a bad idea.

when i place teleport block high above ground (platform high up in the air) i can sometimes see a block texture on the ground , like 100 coordinates lower. after some time the right texture for block is reapplied(dirt)
The thing is - in void word Teleportblock is in y=65 position, in Overworld its in y=150. So... the Overworld block which is in y=65 pos, sometimes gets a texture of a TeleportBlock. Guess in that video its same thing - a block 1 coordinate higher gets a texture, cause in VoidDimension (DIM43) theres a TelportBlock in that coordinates.

1.12 is no longer supported, sorry about this please update to the new 1.14.4 version. If the issue still happens there then open a new issue.

Thank you very much.