When most blocks are placed underwater, they show water flowing down beside them, even though that block is a source block, similar to the behavior of slabs in water.

This seems to mostly affect transparent chisel blocks, though minecraft glass and stained glass does not behave the same way.

Could you please show us a screenshot?

Hello?

This is a vanilla bug, they have glass hardcoded not to have this effect, but any other transparent block will do this.