I want to use the 1.12.2 client to play the 1.18.2 server through this mod

I think it's caused by viaforge mod loading Mapping before the mod is loaded

Related Links：
ViaForge: https://github.com/FlorianMichael/ViaForge/tree/forge-1.12.2

I'll see what I can do.

ViaVersion is generally not compatible with this because it decides at protocol level which blocks exist in which version and then exchanges them, so the mod never gets the original data from the server.