I am unable to reproduce this issue in the way you stated. Could you give me your exact dialogue choices and steps used. Thanks!
I don't know the exact dialogue choices (that would require me reloading a save around that point, which i don't have), but i can tell you approximately the steps i went through.
The quest was initiated when i approached the grain mill. I saw the villagers arguing with Trumbel, and then march off. I immediately went inside, spoke with Trumbel, and caught him lying (i believe there was a resolve check involved, i remember the required stat number being 14) about not having enough grain. There were other stat check options available afterwards, but i don't believe i had enough in said stats to proceed with them, so i believe i used a diplomatic option to convince him to share his grain (something along the lines of "Is it worth putting your family in danger over this grain?").
Afterwards, the conversation ended with him telling me to go inform Sweynur, and generally being grumpy about it. I didn't immediately go do that, however, because i didn't know where Sweynur was. I went through the Temple of Eothas, completed its associated quest, and then went to to inn and stumbled upon Sweynur.
I initiated conversation (at this point i had some sort of modifier for speaking with Gilded Vale townspeople, i think, because next to his name while he spoke was a phrase indicating friendly reputation) and i believe asked him what was troubling him. He told me about Trumbel, and i had a diplomatic option appear to convince Sweynur and his buddies to back off of the miller(i believe the option was something along the lines of "Don't you think you've been hard on Trumbel? It's time for you to rely on your fellow farmers now" or something). On the screen of dialogue choices i also had to option to simply tell him that Trumbel agreed to lower prices, but i figured it couldn't hurt to go with the diplomatic choice for my paladin order. So he told me to go inform Trumbel that they'd lay off him, and the conversation ended.
That's where the problem is, as i cannot initiate conversation with either Trumbel or Sweynur. Both of them simply have floating text when i click on them, as if they were ordinary villagers now (though it is contextual, with Sweynur saying "You've gotten what you wanted, what else do you want with us?" and Trumbel telling me that the grain won't last much longer now).
Sorry for not being able to get any more specific, the only save i have near this point is in the Temple of Eothas, about halfway through the second level of it, which is after i convinced Trumbel to lower prices, but before i spoke with Sweynur.