could be a stealth change by blizzard, intentionally or as a result of the 20-29 bracket change? where vet20s are being treated as f2p20?
XP past 20 was banked for f2p20s up to level 30, so if you made your trial account active every f2p20 would pop past 20 up to 30 max.
as for vet20s, they leave their XP on when their account went inactive, but never gained XP while inactive and until recently were also considered XP on for BGS.
i would test what you are saying but i took the free 50 boost and SL upgrade and have recently used a token. my vet20s were last played in the template era of the legion expac