Wouldn't call this an actual 'issue', as it isn't crashing the game or causing any major problems, more of a minor observation.

Over time, regardless of the amount of time between each conversation one has with a villager, and regardless of how good or bad the mood of the villager in question is during those conversations, the percent chance of having a successful interaction (any interaction, including Chat, Joke, Tell Story, etc. with the exception of Gift, which is always 100%) with any villager dwindles down with every interaction to 0%, making it impossible to build Heart reputation with that villager using any other interaction choices besides Gift, which eventually ends up only giving 1 Heart point at this point, even when gifting them Diamonds or other valuable things of that nature.

The percentage layout they adopt at that point is similar to that of any villager in a Mad or Seething state, where all interactions besides Gift have a 0% chance of success, resulting in negative Heart reputation if one engages them in any mode of interaction. They do this in any and all moods, even Cheerful and Ecstatic.

Are villagers designed to become increasingly difficult to interact with each interaction, or do you think there's something off?
Can provide a list of other mods and plugins and a server description and all that if you think it's necessary. On the whole, the mod is operating 100%, by far one of the most well-done mods I've seen.

Villagers do become more difficult to interact with after each interaction (interaction fatigue), but their interaction fatigue resets about every 30 seconds or so.

Interaction fatigue influences how likely it is for the interaction to succeed. Their mood and trait contribute as well.

Here's how "Chat" is calculated:
base chance of success = 65%
chance modifier = -(number of recent interactions * 7) + mood modifier + trait modifier
success probability = base chance of success + chance modifier

The success probability is what is displayed on-screen as a percentage.

I've checked that interaction fatigue is resetting correctly, but if you continue to see this problem, I'll investigate it further. Interact with a villager and wait about a minute - see if their success percentages increase back to normal.

Keep me updated.

Ah yes, think I discovered the root of the problem.
Had 'gamerule doDaylightCycle' set to false for building purposes, so actual time wasn't passing in-game, meaning that all the interaction fatigue accumulated by the villagers was being built up over time without allowing for that 30 second game time reset.

You're the man, appreciate the prompt response as well.