weird laggy "shadow blocks" apear while rapidly breaking blocks near rivers

for some reason while instamining near a river i started getting quite a lot of frame drops as well as this small black thing flashing on the screen

how to replicate:

1: make a new world. or join an old one. doesn't matter

2: get an efficiency V diamond shovel

3: go to a river with a lot of sand in or near it

4: dig the sand to find the flashing blocks