Summary:
In Java Edition 1.16, a change was made so that when fishing, treasure loot can only be obtained by fishing in open waters. This change has not been made in Bedrock.

According to the changelog linked above, a fishing location is considered to be open water if the fishing hook is in water, there are no blocks besides lily pads above water, and all water blocks are source blocks with no solid underwater blocks around.

Steps to Reproduce:

1. Obtain a fishing rod
2. Enchant the fishing rod with Lure 3 and Luck of the Sea 3
3. Dig a 1 block deep hole
4. Place water in the hole
5. Fish for a few minutes

Observed Results:
Treasure loot can be obtained by fishing in non open water.

Expected Results:
Treasure loot should only be obtainable by fishing in open waters.