I’ve run into this now a couple of times - although e.g. my flight plan clearly goes easterly, SimBrief generates an even flight level when left at AUTO.
Not all FIRs follow the East = Odd, West = Even rule. On your route, both the LECB and LIMM FIRs use a North = Even, South = Odd rule. So because you’re flying Northbound, it would choose an Even altitude.
It’s worth mentioning, however, that you had a few step climbs on your flight plan, alternating between even and odd altitudes. This is because the other FIRs along your route do use the East/West rule, so the system had to switch between even and odd altitudes a few times to account for this.