I have a guardian farm where guardians spawn and then eventually fall through signs and down a big hole where they die at the bottom. After the 1.4 update, guardians sometimes float in midair shortly after the start of the drop. There is no water around them, but they stay there and bob up and down.
I went to creative to test this. The same thing happens in a completely cleared out monument. This is the attached picture. In this situation it almost looks like a guardian can "try to swim" toward a nearby water block. Even if it is hanging in the air, it can try to swim so strongly that it floats.
I couldn't duplicate this in a random location, so it may be related to monuments only.