I have a new pair new as of this spring) of Speedplay Frogs. I love them, except one of the cleats has taken to releasing on its own. - but the left has a good bit of play (I can "rock" the shoe side to side when engaged).

I can see no physical difference between the two cleats; the little spring loaded wedge that snaps down behind the ring on the pedal is just as "high" as the one on the other cleat.

Has anyone had this experience?

I rode in Speedplay Frogs for many years and tens of thousands of miles and never had the problem you are describing. All I can think of to be sure the bolts holding the cleats to the shoes are tight enough to keep the wedges elevated properly. If they are ok, then I'd contact Speedplay about the issue. They have a 1 year warranty and it should still be in effect.

Also have at least 50k mi on Frogs and generally have no problems except when installing new cleats, I occasionally have to fiddle with the
amount of elastomer under the wedge. Once done they usually function well until the cleat wears out. I have had your problem on one set of
pedals but it has been so infrequent that I have not investigated. You might swap the cleats on the shoes and see if the problem swaps as
well, and you might try adding a smidgen of thin rubber under the offending cleat wedge if the bolts are not loose. Once I was unable to unclip
on a Frog when a cleat retention screw loosened up and jammed the cleat in place. I have also found screw tension affects the elastomer springiness
also. The blue goop on the screws is good for the initial install and should be supplemented with more threadlocker if you adjust the screw.
Here with the very small screws using blue locker is ok, small screws are unlikely to be frozen by the "permanent" blue threadlocker.

Blue Locktite (Grade 243 and it's competitive clones) is not "permanent" and bolts, unless very small, using it can be removed with just hand tools. The Speedplay cleat bolts are M5 and Blue Locktite is fine as it will keep them in place but allow removal when you need to.

My best guess is that the alignment of one cleat is off a bit, such that your foot and ankle rotates just enough to disengage during pedaling. If you can, you might aim that cleat so that your foot can wiggle through a crank rotation without releasing the cleat.