COMPRESSION SET C050
The elastomer (rubber part) changes shape when it has been exposed to too much heat.
Any time the temperature of an elastomer is raised to its curing temperature the elastomer will conform to the shape of its mold.
O-rings are located in O-ring grooves that are machined square not round. That is the reason we sometimes put in a round O-ring and take out a square one.
The change in shape is telling you the O-ring got too hot.