Thorns enchantment work even when you hold an item in your hand

I was unable to reproduce in Minecraft 1.16 pre-release 5

Your video appears to be corrupt, and I don't quite understand what your issue is by only reading your description.

Re uploaded

You don't have the thorns enchantment on any piece of armor you have equipped. Enchantments will only take affect if you have the armor equipped.

This is my problem! Enchantment works even when the item is not equipped.

Ah my bad. On that case can you send a video of your friends screen? It's hard to see the damage effect from your recording

This actually appears to be a duplicate of MC-73710, which hasn't yet been resolved. If you have any additional information to provide regarding this issue, feel free to comment on the parent ticket. If you feel that this issue is affecting you, feel free to vote for the duplicated ticket, to give Mojang a basic idea of how many people are impacted by the problem. Just to further inform you, this will not determine how quickly the bug gets handled or fixed.