SPD pedal float becomes restricted
(This is more a collection of thoughts to help youtowards an answer, than an actual answer)

If wear at that point causes the shoe to sit a touch lower in the pedal, and the rubber sole to press against the metal, you'll get extra friction.  My shoes are wth me, and show a similar wear spot; perhaps on your new cleats it's just not yet obvious.  And that spot might actually contact something.  I could believe so from about 4:46 into this video (M324s, but the mechanism should be very similar) and also from my own shoes (used with M424s)

Because I ride on M424s comnbined with walkable shoes, I had to file bits off the pedal to stop them binding against the shoes.  To find where they were binding I dampened the sole and dusted the pedal with coloured chalk, then clipped the pedal (off the bike) to the shoe held between my knees.  I could then see where the chalk stuck to the shoe. You might try something similar, though it might be easier to see where the chalk (alternatively talc) rubs off the shoe.  You may not be able to make the problem go away with the file as your pedals don't have much material, but a light trim of the sole in the right region might help.

Which foot do you unclip more?  The one that shows the trouble?

You can also try throwing some light grease over all the metal parts.