The reunion of Thessalonica to the larger empire of Nicaea did not eliminate the Comnenus-Ducas family of imperial contenders. Michael II was the grandson of the first Michael, who started the whole despotate of Epirus, as well as the nephew of Theodore. Probably on account of Theodore, Michael returned to the original spot where his grandfather had started his resistance movement realizing that Thessalonica was lost in all but name under John Asen's thumb. This new splinter group prospered enough to give John III of Nicaea headaches. He, of course, had been planning to retake Constantinople for years and now that Thessalonica had been regained Michael's motley band of dissidents was more a nuisance than an obstacle. Rather than fight a protracted battle with elusive guerillas based in a mountainous region, he chose to recognize them, for the time being, and set his sights back on Constantinople.
The truce was broken some years later at the behest of Theodore, still the frustrated hopeful, but his forces were unable to rout a detachment sent by John. Another stalemate was evident so another treaty was signed and this time both kept to their own sides (for a few years anyway). Michael would linger on for another twenty years in his Greek foothold. After his death hopes of the province returning to Constantinople faded as his descendants, pretenders and various enemies of the united empire kept the two forever divorced afterwards.
Unlisted SB, DOC III 3a-b AE Follis Obv: Busts of Michael and John.
Rev: Walls of Thessalonica.