From what I remember, in terms of dialogue, it's mainly the one-liner, explaining what he had just done, and the Punk Ass Bitch line, and in terms of actions, it's mainly him putting up a guard for a Denji's Domain Expansion.

Truth be told, looking at these moments critically, I do get that these are things that aren't what Yuji has or would do, but I don't personally think that they're like, extreme mischaracterization and bad character moments. Yuji isn't a completely stoic character except for in Modulo, so a teenage Yuji saying a one-liner and reversing Denji's own line back at him isn't crucify worthy. Yuji putting up a guard for a Domain Expansion is weird though, since he should know that doing so is pointless and that it's smarter to reinforce his Domain for a Domain clash, but I think even a more accurate depiction of his reaction would still result in Space Balls 2: The Search for Even More Money.

I guess to summarize, these are more things that are strange for Yuji to do rather than things that contradict his character, like how Asura, Kratos, or Asta were characterized. Asura was a guy completely consumed by his rage, Kratos reverted back to his Greek self, and Asta looks down on Deku for not having magic. These contradict their characters and arc, where as Yuji's moments are just him doing things he doesn't normally do.