No, it's not a bug and has been like that since 0.9 (or maybe even 0.8).
The type of bandits is random. It can be anyone from a pretty large list.
Sometimes it's brigands, other times it's knights of doom or, say, gladiators.
Sometimes it's slave girls.

I'm not sure if it's a bug or a feature, the villagers in taverns will tell you bandits attacked but in Perisno the "bandits" are often high tier units. It just makes bandits a little more challenging to clear out.

its not a bug. can be a bit of a nightmare though if youre riding up expecting an easy fight and get doom knights XD

引用自 Brendan Fraser's Career：

I'm not sure if it's a bug or a feature, the villagers in taverns will tell you bandits attacked

Hmm, it's more like we never really thought about changing it.
The "bandits" the villagers have always been talking about are just... you know, bandits. In general.
But I guess it would make sense to specify the type of attackers in the dialogue, so I'll probably do it.