It's more coherent then is being made out to be. His progress isn't linear or or easy. The recent hangup he had with magic was still consistent. What he could do then was still more than what he could do when he was hiding his ability. It's just that proceeding beyond that point took more than what his character development had shown up till then. So, they had a character arc that was devoted to him thinking more about what he wanted to be. Up till then, his goals had revolved around getting him and his family up to being commoners, about becoming equal in the eyes of others. He eventually had to start thinking about what he wants to be once free and who is important to him. So, there was progression, he didn't slip backwards in terms of power or character. It's just that people latched onto the notion that he has talent for magic and assumed that that will mean he stays ahead in that area but others caught up and surpassed what he could muster with his talent and restraints.
For the mc's attitude, he does change and become more self confident as the story goes on. He learns to fend off his treatment and assert himself more. Doesn't mean he'll pick a fight with Evan over anything he mutters under his breath though he'll definitely try to best him in any match they have now.
Now, for chapter 28 that has everyone the most frustrated...I don't think it's real. It went by too fast like it's his life flashing before his eyes and it is too far detached from the route the story was going, dismissing everything he's built up. More so, the way the last panel looked on chapter 27 suggests that he might have lost consciousness. This is probably a hallucination he's having about what could go wrong with him losing everything, being reduced to just being a slave again and everyone suddenly treating him the worse than they ever had. This is probably just a look at the mc's fears, not what's actually happening, which is why it feels so disconnected.