It's a lot harder to take on an offensive role in the NHL than it is in the AHL as a defenseman, moreso as a rookie, doubly so as a rookie with a relatively short leash (Fleury got scratched after two games and his icetime gets cut severely when he performs a bit more poorly, so there's your evidence). Given these facts, I wouldn't say that part of his game has regressed much or at all this year, it's only more difficult for him to exhibit his offense given his inexperience, the pressure on him and his diminished role.
Not just looking at stats but watching Fleury play you can see that he still manages to get good shots in from the point (but they haven't gotten in or got delfected aside from last night), keeps up with play and passes the puck generally well while actually being pretty good on the breakout and transition (sprung Byron and the like a few times on quasi-breakaways earlier in the season but they did nothing with it each time). To reach his offensive ceiling he just needs to calm down, adjust to the speed and pressure a bit better, execute a bit faster and have the poise to find some open options on the offensive side or when dealing with pressure. Without experience and playing under duress and learning from it, it is very difficult to improve in those aspects, so while a trip to the AHL might rekindle Fleury's confidence a bit and light a fire under him, I think that playing games for the Montreal Canadiens in tight situations is better for him in the short and long term since he isn't really struggling at all, only going through the same growing pains that every defender (save the Raymond Bourques of the world, but they're damn rare) goes through to begin their career in the NHL.
However, if there ever comes a time when Cale Fleury looks overwhelmed and struggles for more than an odd shift here and there, then yeah, he should get sent to the AHL to work on his game and confidence. In that eventuality, we'll be able to bring up Juulsen or play Folin in his place with not much loss at all, but until then Fleury has shown to be good for us and deserves his icetime on the team.
With more time and experience at the NHL level I wouldn't be surprised if Fleury eventually finds more of a scoring touch and manages to produce more consistently. Even then, having watched him play a lot these last three years, I don't think Cale Fleury has much more than 30-40 points potential in a season in the NHL at best. If he develops to his full potential, we'll have an extremely solid two-way top 4 RHD on our hands.