The Asobo stock A320neo has one type of FMC, the original one from MSFS. The LVFR family uses an enhanced version of the stock A320neo. The FBW A320 uses different coding by 3rd party developers for their FMC. Although it looks the same, it works differently. The 787 and 747 FMC were created by Working Title and integrated into MSFS.
When MSFS was first developed years ago, flight plans were supposed to be created using the World Map before the flight. To keep things simple for most users, airways were not included in the World Map. They still are not available in the World Map. Since the A320neo and the World Map have no import capability from SimBrief, Navigraph, or other apps, there was no need to include airways in the A320neo or in the World Map. This isn’t a bug but rather lacking a feature.
The World Map has been partially enhanced to recognize airways in any imported flight plans by “unrolling” them into there corresponding waypoints. The same thing happens with the 787 and 747 FMC. The FMC has two keys, RTE and LEGS. The only place to enter airways is on the RTE menu. The FMC takes the airway and “unrolls” them into their corresponding waypoints on the LEGS menu.
I hope this explains why for some airliners, “NOT IN DATABASE” is displayed when trying to enter airways. The only “workaround” to manually insert an airway is to input the corresponding waypoints for that particular airway segment.