Well, it's not a bug insofar as if Teudogar dies, his death would usually be a result of your actions (because no one else would act against him on his own accord).
And after Teudogar's death, the Alliance-with-Rome plot is no longer viable (because without Teudogar's influence, Ortwin and his supporters would always prevail in the popular assembly). So it wouldn't make any sense for Teudogar's henchmen to continue training you.
However, it's a bug insofar as the game doesn't differentiate between who or what killed Teudogar, but simply assumes your guilt. And in your scenario, you really aren't responsible.
So I think it's right for Teudogar's clan not to cooperate with you anymore after his death; but in cases where his death wasn't your immediate fault, they should still treat you civilly, and of cause, his henchmen should not 'avenge' his death on you.
Internally, that'd require adding a You-killed-Teudogar flag to the current Teudogar-is-dead flag, and considering both in all dialogs and plot triggers that currently use only the latter. The problem is that it's a bit tricky to reliably determine your guilt in a number of borderline case (like, e.g., poison, or, let's say he's wounded and dies of bleeding, possibly after you already made peace with him, and so on).
In other words, every relaxation of the You-killed-him check could result in a number of scenarios in which you'd be clearly guilty going unpunished. That's why I think it's generally better to check for his death only, and leave the precise cause aside, even if this means you're sometimes punished in cases where you're technically innocent.
