It sounds to me like a very tight seal? So the tightness creates extra friction. The extra friction creates more heat leading to seal expansion. Even tighter seal causes even more friction and on and on we go......?

I'd be checking the seal fit and, if tight, size accordingly. I've never felt the need to size one, preferring to let them run in. But, depending on the scale of the power loss, sizing might be apt here?

Or....have we used a low grade, non-scragged spring and, as it goes through a few compression and expansion cycles, is scragging itself and settling????