This has been a longstanding bug that should be fixed given the release of Exalted weapon modding and the addition of new mods that impact many Warframe/melee builds.
Applying Forma to a gear item locks your "XP" statistic on your profile until you reach that value + 1 from 0 to progress this counter. What does that mean? Well here is my test to display the bug.
Here is Equinox:
Equinox applies one Forma and proceeds to level.
410,985 Affinity is earned from a mis