Yes its true, you need to take the measurements when you are completely flaccid and fully stretched.
If you ordered the size according to your semi erect condition it will slip off as you shrink to the completely flaccid state. Same with setting up the Corkscrew, once you start to lose your semi you will lose the secure grip and slippage becomes and issue.
You could experiment with setting up a tighter fit with the coils at the base of the shaft to help maintain engorgement.
Some guys order two sizes, one for regular wear and a second size for maintaining post pump engorgement.
I am just curious as the the size you are wearing now as the dynamics can be quite different depending on the size you are working with.