Why is only one particular trade of my villagers getting more expensive?

I'm currently playing on my own Minecraft server and have my villagers there, all of which I zombified and healed twice. In the beginning everything was great and they only asked for 1 emerald for each trade (with a few exceptions), but now exactly 4 of my 12 villagers are becoming more and more expensive. But not as normal that they are generally more expensive, no, only the coal trades are more expensive with my armaments smiths, with my clergy only the warts trade and with my librarians only the nametag trade. In advance: It can't be because I accidentally hit the villagers, their iron golems, or their cats, and it can't be because I don't deal enough with them. I've already suggested that it might be because I'm only making one trade all the time. I have already done this with my villagers who have already died and nothing got more expensive there. In addition, not only would a certain trade become more expensive, but all of them. Can you help me please? I'm downright desperate.

This is completely normal. The villagers "level" themselves through purchases… So it is that the trade that you use for leveling becomes higher, since you also unlock other offers or better offers.

Thank you, my villagers are already max level and I don't use the warts or the nametag trade to trade a villager up.