I'm not a spokesman. I've been a customer of BCY's for the past 20 years or so, and used several pounds of B-55 over the years.
Never heard anyone say that B-55 had more stretch vs. B-50 before. BCY has sold over 2,000 pounds of B-55 just this year, and yours has been the only complaint. To the contrary, gobs of customers have reported just the opposite.
If there was a legitimate problem, they tend to think there would be someone else noticing it. They haven't become the #1 bowstring material manufacturer in the world by ignoring problems or customers.
You neglected to mention that the owner talked to you himself, referred you to me to try and help, and even sent you a new spool for free. Even though you told them you had always used B-50 and bought the ONE spool to try, they treated you like you were a million dollar customer. Try spending less than $10 retail and see if you can talk to the owner of any other multi-million dollar company. Instead of showing appreciation for or even mentioning their effort, you remark that they "claim" it has less stretch. That's how BCY works, and one of the main reasons I'm a loyal customer.
I don't know what the problem is, but the simple fact is B-55 is a higher grade of polyester. The material itself has a little less stretch and creep vs. B-50--certainly not more. The only other big variable is the build. I know you said you have been building strings for a long time. I've quit counting how many I've made at 10,000 years ago...and I'm still learning something new all the time.
Finally, neither material will solve the O.P.'s problem, as any polyester material is going to stretch and creep, even moreso in warmer temps, so it's a moot point.