Aircraft:
Headwind A339X with correct airframe in Simbrief selected, pax 373, cargo 7460kg, block fuel 53984kg (I rounded it to 54000kg)
Route with generated step climbs:
MMUN/12R N0475F350 TAKU1A TAKUX UM782 OMIRO UN420 TBG/N0473F370 UN420 ROLUS/N0471F380 UL300 IQT/N0469F370 UM776 OPTOP/N0465F390 UM776 JUJ UT672 MULTA UW24 SNT SNT7U SAEZ/11
Issue:
Simbrief suggested step climb to FL370 at TBG… MCDU gave warning that step altitude is over REC MAX… checked that and REC MAX FL360, OPT FL330. Definitely not able and/or safe climb recommendation.
Maybe airframe or something needs updated in SimBrief? But as I dunno responsibility for this issue so let’s start bug reporting here. 
Update during flight: MCDU gave OPT FL370 / REC MAX FL390 approx 30min after passing OPTOP where should already be climbing to FL390.
Hi, hard to say which is correct since data for the A339 is hard to come by. I don’t know where Headwind took their figures from, though I’m a little surprised. I would expect the A339 to be able to climb higher than the A333 for a given weight.
Either way, it would be up to them to update their official SimBrief airframes (at the bottom of this page: A339X - A330-900neo - Headwind Simulations) to offset the optimum cruise altitude to match their FMS. Based on your report, maybe a Cruise Level Offset parameter of M2000 or M3000.
Best regards,
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I will contact Headwind next, thanks.