IIII FLavia firma- source for epithet Firma ???

[Roy]

And all of a sudden I realized that besides few respectable authors and some unreliable internet sources there is no evidence for giving the IIII flavia an epithet firma.

Most of those internet sources state that IIII Macedonica was reconstituted under the name IIII Flavia Firma.

IIII Macedonica was reconstituted as IIII Flavia Felix.
XVI Gallica was reconstituted as XVI Flavia Firma.
Both were reconstituted, and got the new name, after the Batavian rebellion.
So, any website that mentions IIII Flavia Firma probably mixed up those two legions