Polyester/dacron (B-50 and B-55) is very elastic. I can easily stretch a string 3" or more on my jig, just to have it retract when the tension is removed.
All materials will retract to some degree when you unstring the bow--how much depends on a lot of factors such as: draw weight, strand count, type material, type string, how many twists are in the string, temperature, etc.
When you brace the bow, give it a little time or just shoot a few arrows to settle it in.
Factors that have the most effect on string stretch are: type material, draw weight, strand count, and temperature.
The most elastic is polyester/Dacron. Heavier draw weights will cause the string to stretch more when strung (thus retracting more when unstrung), as do lower strand counts. Materials stretch more in higher temps.
Chad