some quick test in RB 6.6.3.
So when I'm not looping at all, just playing tracks at 126 bpm, they stay in sync.
When I loop without sync, they slowly drift apart. Using sync seems to help to keep them on beat. The shorter the loop (like 4 beats), the quicker they drift apart. This is probably why I never really noticed as If I do ever loop, it's usually 2 to 8 bars.
In 4 deck mode, there was some observations I made while running 4 and 8 beats loops:
deck 1 + 4 seems to stay in sync together without SYNC
deck 2 + 3 seems to somewhat stay in sync together without SYNC, but slowly drifts off
deck 2 seems to drift off (slower) than deck 1
deck 3 seems to drift off (faster) than deck 1
deck 2 seems to drift off (slower) than deck 4
tl;dr version: looping on deck 1 and 4 together seems to be more stable in my version of RekordBox on Windows 10 Pro (the 4 beat loop went on in beat without sync for 20 minutes). Absolutely no idea why. Looping on other decks drift off for me within 1 minute.
Seems like there is a problem inside RB to keep all the decks in sync when looping. Perhaps a race condition?