When connecting to a 1.12.2 server using a 1.15.2 client (or 1.14.4, with the respective EasierVillagerTrading and fabric mods) the mod does not work. Clicking the GUI (or shift / ctrl clicking) causes the trade to appear for a split second before returning the items to my inventory. Occasionally, 1 or 2 trade "transactions" will go through, but very rarely.

Additionally, the trades with still show as being locked out after a few attempts, even though no trades have happened. This seems to be only a visual glitch, however.

Is there any way this could be fixed in the future, or is this a problem with ViaVersion?

What's sent to the server on villager trades has changed considerably between 1.13 and 1.14, because up to 1.13, you could only change the current trade step for step, 1.14 lets you change that directly. EasierVillagerTrading sends what a 1.14 (or 1.15) server needs, it might be that ViaVersion translates that incorrectly when packets come from EVT compared to packets from normal play.

However, EVT does its work by simulating clicks to the GUI, just like what a player would do.

However, ViaVersion's docs say they have a packet limiting system that's there right out of the box; I'd rather expect that to be the culprit. Sending a lot of clicks in a very short time could trigger that system; I know servers that use packet counters like this in Anti-Cheat-plugins think clients with EVT are cheating and reject its transactions.