There are no bugs with this but may need to try to explain it again. The bug vamp is talking about lasted for maybe 2-3 weeks. If the tenth buy is the first double drop and you buy a full buy, reset, and then buy another ten you will only have unlocked 40 raids with x2. If it was under a x2 event you will have those 40 raids to use after the event ends. If you buy them without any x2 running and first buy 10 and raid something your 20 raids will be used up fast. Buying stamina 10 times while not under a x2 drop is very inefficient though and I would never recommend it.
Using a stamina reset will reset the progress for when you get free raids. If under vip around 17 the x2+ stamina feature is rather useless. You don’t get enough buys and won’t have the diamonds to buy enough raids to really utilize it.