there is a bug that sometimes a surplus of XP won’t be credited for a new module or vehicle.
When you search for that bug you will find topics like this: Community Bug Reporting System
But this link seems to be a russian overview of forum topics.
As the conversation in the issue topic is closed I cannot ask about the current status.
Maybe someone can tell me here in this forum if that bug will ever be fixed?
I’ve been playing this game for almost 12 years and I experienced this bug very! often.
You can call it a bug if you wish. If you understand what actually happens, you can avoid it or make it work to your advantage.
But you sound as if you have not yet figured out how the system works.