The wiki says:
Originally Posted by AKronblad
I do think that if a tag change is involved with your question that a save and full restart could fix the problem as this is the only way to get the game to correctly reset things due to the tag change. Until this is done people have reported strange things with cores etc etc etc that all go away and are fix by the full restart. If you are still worried about your trade goods on reload then make an event based on regions to put the trade goods back on your empty provinces.
The military system is based on a model of "preferred units" for land units. Core provinces will produce preferred units. Owned provinces that other countries have cores own may recruit the preferred units of that country (even if from a different technology group). All the units already built will be automatically changed to the new preferred one, except for mercenary units.
If a tag change is not involved then most likely the preferred units is the cause.