Pockets of flowing water generate in areas of the underwater temple, such as along the edges of the long hallways that encircle the inside of the temple. These are rather annoying when the player gets pulled into them.
Linked issues
Comments 2
This should be forward resolved to MC-144929 since it describes this issue more accurately.
I don't think it's a problem with the monument. it seems to instead be a problem with water flow. Water sources seem to flow near ceilings in 19w08b, causing this issue to occur in the monument. This issue also happens with water sources in underwater caves/ravines, shipwrecks, ruins, cave buffet worlds, and manually placed water. The problem is that water sources will decide to flow in cases that they shouldn't, making it seem like there are air pockets in the monument, when in fact the monument is completely filled with water.