Were you using Unity 6 URP? I also had an issue with decals which were important to the game. I found out that part of the problem was that Unity 6 uses forward+ render by default (at least for URP, maybe others) and that borks the decals in webgl and causes a bunch of other issues. It barely worked at all in Firefox. Switched URP to forward rendering and set decals to screen space and now it's all good.
https://gongylus.itch.io/the-vaults-below-durn-a-field-study