How to Mirror Weight Painting Multiple Objects with One Armature

TL;DR A single rig with multiple objects wont mirror the weight paint when a change is made to a mesh. Have an old file that mirrors weight paints perfectly, edited some meshes and UV stuff but kept everything symmetrical, and now it no longer mirrors. how do I make it mirror again?

So I started working on an old character mesh I made years ago. Its a character composed of many objects but one armature. (Different objects for head, biceps, torso, etc). The step I was on was weight painting and it mirrored perfectly (when I painted on the left bicep mesh for the left bicep bone it painted exactly the same weights on the right bicep mesh for the right bicep bone).

However, I noticed annoying issues with the UV Unwrap and some other odd things so I ended up fixing these things which took hours and ended up deleting some meshes and duplicating them on the other side because I wouldn’t be able to exactly replicate the changes without there being slight differences plus it took almost a whole day for just the left side of the character and its the same exact thing needed for both sides. But now when im ready to pick up where I left off and begin weight painting, it no longer mirrors.

I read posts about meshes not being exactly symmetrical or location applied or symmetrical in location which can cause issues for the mirror. so I double checked almost all verts positions and compared the location to the verts on the other side and everything I changed checked out to be the same. Still wouldn’t work so I deleted all the meshes on one side, every mesh and the armature has an origin of 0,0,0 then duplicated them all and set scale to -1 on the X axis then applied the rotation, scale and position. So now the scale, location, and everything is exactly even on all meshes (Middle meshes mirror perfectly in edit mode so things in the middle are symmetrical as well) and everything has an origin of 0,0,0. Still wont mirror.

I checked every mesh has the x-mirror check box under options ticked and under symmetry mirror its checked “X” and it still wont budge. I don’t understand because if I open up my old file and start weight painting one object it paints the other side. I cant remember what I did years ago but it worked and still does, but if I edit 1 mesh, even if I duplicate it back and make everything symmetrical again, It no longer wants to mirror when weight painting. I don’t want to redo all the work and just pray I don’t accidently move a single thing or delete something just to have the mirror removed. Also I cant make exactly make the same changes on each side without editing or deleting a mesh, I always model and make changes with an X symmetry because inevitably there will be differences that will cause issues down the line.

Also there is no mirror modifier applied to any mesh in the modifiers tab. all meshes appear under the armature so everything is parented.

Is there a setting or something I missed? does something need to be parented in a specific way? I know its possible because I have a file that does it but idk what is changing. I’ve looked at a ton of posts about weight painting an mirroring and nothing has helped, looked up tutorials of people starting models from scratch and none mirror their weight paints. Im at a wall here.

Thank you for any responses in advance!

If your meshes are symmetrical, just delete half and leave a live mirror modifier (above/before the armature modifier) with vertex groups enabled on it.