It could be due to the water movement-Some of the Long fin males can get stressed and some tolerate it without any issue. Some will tolerate it for months and once they mature a bit more-they can start having problems with it.
IMO/E-as males mature they sometimes will not be very active to start with. Sexually mature males normally don't swim non-stop or in constant movement-unless they have a reason-like in view of another fish-especially another Betta, when they swim around to explore-either looking for food or invaders or at feeding time or when you approach the tank or if something is wrong. This species being a surface dwelling, nest builder that establishes territory-tell us that they shouldn't be in constant movement by nature, however, they should swim about on occasion-just not all the time.
How did you re-acclimate him to his tank after treatment