Suspicious blocks in water still give you loot while being brushed when the player is looking at and brushing one at first, but then the player moves his aim to a seagrass next to it when the loot isn't completely brushed out, and then the loot came out as the brushing progress ends. So logically the player is brushing the seagrass but not the suspicious block behind. (as long as your crosshair is aimed on that suspicious block)

Steps to Reproduce:

1. Find or build a structure which has a seagrass next to a suspicious sand or suspicious gravel (Do /setblock ~ ~ ~ minecraft:suspicious_gravel{LootTable:"minecraft:archaeology/trail_ruins_common"} for example, if you choose to build one)

2. Brush it, hold right click and slowly move your aim onto the seagrass

3. See if the suspicious block drops loot successfully.

What I expected to happen was:

Brushing progress stops when there is a seagrass in front of it

What actually happened was:

Brushing progress didn't stop