About Upgrading Iconic Weapon

So after update 2.0, the Iconic Weapons can be upgraded into Tier 5+. Then Phantom Liberty released and those can be upgraded to Tier 5++.

However I notice that some of them got downgraded back to Tier 5 in my upgrade menu. Is this a bug and does anyone else have this problem?

EDIT: I find reloading the save helps with some of the Weapons, but also found out another bug is that Weapon stay the same Tier after upgraded.