Villager trade does not increase villager XP

Hi, when I tried trading with this Fletcher villager, the XP bar does not progress. I'd like to progress him to a Master trader but it does not look like it is possible. The issue also affects other villagers as well, but not all. In the photo, I have traded a few emeralds for a bow - the highest tier trade possible (because I know doing tier 1 trades gets lower XP).

What might help is the fact the world (and the village) was found before version 1.14.4 from what I can remember. I traded with this villager before 1.14.4 too.

Thanks!

Edit: after playing around a bit, I found that only villagers who have empty XP bars (whatever their level) AND who I hadn't traded with for a while would have this issue. i.e. I upgraded another apprentice fletcher who already had XP, and doing the next tier trades gives the fletcher XP as normal.