This bug was also in 1.13 and has carried over to 1.14.

When Guardians spawn in their water source, and the guardian then moves out of the water source block into an air block (as in guardian farms) instead of falling to the ground the guardians fall about six blocks and then float in mid air,they also will not travel away from the water source block more than about 6 blocks if they are in a stream of water, after a lot of testing I have discovered that the guardians (roughly 25% of them) are trying to return to the spawn location, the only way to stop the guardians from pathfinding to the spawn is to drop them through some lava before they reach the 6 block distance from the spawn, although if the guardians pass through or travel in any other water they will automatically start pathfinding the spawn again.

If you wish to see screen grabs of floating guardians (on mass) look at my screenshots on xbox one, Gamertag Army of Strongs

Ps also fix the spawn rates of Enderman in the end (they have dropped significantly in the end (in remote platform enderman farms) Mine is now useless, gone from repairing my pick axe in five minutes to one hour, unacceptable.