Summary:
If you add or remove the loyalty enchantment from a thrown trident using the /item command, this change won't affect the tridents behavior until the chunk of the trident is reloaded. This means if you add the loyalty enchantment to an unenchanted thrown trident, the trident won't return to you (or the drowned) until the chunk is reloaded. Removing the loyalty enchantment from the trident won't prevent it from returning unless the chunk is reloaded. It doesn't matter whether you change it after the trident lands or before it lands. Relates to MC-226344 and MC-127321.

Steps to reproduce:

1. Throw an unenchanted trident.

2. /item replace entity @e[type=minecraft:trident] contents with trident[enchantments={levels:{"minecraft:channeling":1,"minecraft:impaling":5,"minecraft:loyalty":3}}] 1

3. Wait for the trident to return to notice that it doesn't.
4. Relog.

Observed result:
The trident does not return to you until relog.

Expected result:
The trident should return to you as it was replaced by a loyalty enchantment

Steps to reproduce:

1. Obtain a loyalty trident.
2. Throw it in survival mode.
3. Run the following command quickly:

   /item replace entity @e[type=minecraft:trident] contents with trident

4. Notice that the unenchanted trident returns to you.
5. Obtain another loyalty trident.
6. Throw it in survival mode again.
7. Run the following command quickly:

   /item replace entity @e[type=minecraft:trident] contents with trident

8. Relog before the trident returns to you.

Observed result:
The trident is only prevented from returning if you relog.

Expected result:
The trident should be prevented from returning even if you don't relog.

Note:
The impaling and channeling enchantments are not affected by this issue.

Video:
loyalty_command.mp4