20000, 22000, 24000, 26000, 28000, 30000, 32000, 34000, 36000, etc.
21000, 23000, 25000, 27000, 29000, 31000, 33000, 35000, 37000, etc.
This is more complex than most airspaces, so normally I’d be inclined to trust it. In any case, there is no bug in SimBrief. Only potentially a bug in the AIRAC data from our provider.
Unfortunately I can’t cross-reference with the Switzerland AIP since it seems to be behind a paywall, unless you have access or know of a way we can check the AIP (specifically the cruising levels for the N871 airway)?
For what it’s worth, the SimBrief database does contain the proper restrictions (airways T50, Z669, and N871 all use a reverse flight level assignment, so westbound should be odd levels and eastbound should be even, max level F195).
As far as I can tell (very limited experience reading airway charts), document doesn’t appear to specify east/west and north/south rules, but rather, in which direction you’re taking the airway, for example:
UL613, going from HOC to NATLI, even levels, going from NATLI to HOC, odd levels
N871, one-way airway, even levels from MOLUS to DITON, odd levels from DITON to GAMSA
T50, one-way airway (and also one-leg as far as Swiss airspace is concerned), always flown on odd levels
Interestingly, there’s no difference in the level parity whether you are above or below FL195.