I don't know, i don't remember the specifics anymore, but I got the feeling just before it happened, that there was a skill or line in Claude's stats, that says that he will get power when his lord needs it, so this isn't a complete asspull....
Ok, so whatever the ability was it overwrote the slave/weakening curse that was placed on Claude and gave him back either part or all of his original strength.
dem plot armor. MC make bad choice, overestimate himself, eat suspicious thing from unknown people then cry to get plot armor. was good till this plot armor part.
oh well i just hope MC know how to doubt and grow abit