Having being serviced, greased etc when in use approx 10 rotations the platform tightens solid.
Serviced as in axle out, half full with grease and purge. Or serviced as axle out and disassembled? Of the later, two things spring to mind. Either locknut not tightened properly or perhaps there's a tiny ball bearing not quite sat properly on the race.
Or it could just be some grit that didn't purge through.
Have had the same twice when adjusting the axles. Either ends up tightening itself or loosening. And I do know what I'm doing with them, so equally flummoxed. I did the only sensible thing and bought new pedals as that was cheaper than buying the replacement axles.
the outer cone needs to be locked tight against the locking nut. I think the cone uses an 8mm spanner and the locknut is 7mm, IIRC.