Frogs slowly move South East instead of moving around uniformly.
In any given enclosure, after a small amount of time, frogs will start clumping at the South East corner, with very few frogs moving around in other areas, and absolutely no frogs near the North West corner of the area.
Here I spawned a bunch of frogs in the middle of a closed square.
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The frogs then, as expected, started to spread out as they moved.
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But as time went on, the frogs started to slowly move towards the South East corner of the enclosure and ended up all clumped next to the corner.
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Yes, I'm certain 100%. I first encountered the issue in another superflat world I made which was in peaceful mode, where I had a bunch of frogs trapped inside a size 50 world border. Since it was in peaceful mode and the worldborder was so small, no slimes spawned, but all the frogs ended up at the south east corner anyway. That's when I decided to re-create the issue in this second superflat world where I observed the same behaviour, all the frogs ended up at the south east corner specifically.
Note that I did find some new information. This behaviour seems to only appear in terrain that is completely flat without any plants or blocks. Once I bonemealed the grass and there were plants and flowers everywhere, the frogs seemed to spread uniformly like they should. I didn't test this in any other kind of terrain though, only flat with nothing and flat with plants.
I assume that what's happening is that when frogs have nothing to jump on, they jump "randomly" which has a tendency to take them south east. However, when the area is full of plants, the frogs prioritize jumping on the plants, and since the plants are everywhere, the frogs spread everywhere. I noticed this too when I placed a single block in the middle of the area, the frogs would prioritize jumping on the block as opposed to jumping on the flat floor, and so a small group of frogs would remain clumped around the block while the others spread like in the pictures above.
So I guess the way to fix this issue would be to tweak the "randomness" of where frogs will jump if they have nothing else to jump on, since right now it seems to have a tendency to take them south east. But that's just my best guess.

Confirmed in a world with mob spawning disabled.
Are you certain that the nearby slimes within the superflat world are not hindering the results of your findings?