The achievements don't keep what you have earned for achievements from the mod. You keep getting the same ones when placing down each one that had an achievement behind it. The only true one that stays on the server info for who got the first achievement is "You've Been Installed!" How can this be fixed?

What version of Minecraft and my mod are you using? I remember fixing this a long time ago.

Tis 1.7.10 minecraft version and as well as 3.4.7 for your mod. It seems to happen every time reboot the server. Also when getting the achievement for any of them, it says "invalid achievement" or something like that on chat.

※If I do something break the rules or impolite, please tell me asap.
Though I'm just a rookie here, I finally made a fix for 1.7.10 server's achievement bug, which just ported newer repository code to 1.7.10, and only replace a class file in jar file; also tested it will do a correct registerstat() and normally display like screenshot shows.

extract this archive from below, and drag into v3.4.8 (1.7.10)'s jar or corresponding location inside to fix that.
https://drive.google.com/open?id=1XR5AWgNBtf_AB5RbYQdC_UfSbsCepfmM
2023.11.22 link updated where old link dead:
https://drive.google.com/file/d/1srrkVJsijebwYPejXk5cEKC5J0Aj0WYc/view?usp=drive_link
Fix on server side only won't affect client playing, while achievement will state invalid in client, but actually working in server side.

enjoy :)

btw, love MrCrayFish's furniture mods very much <3