Can't make weights symmetrical

Hello!
I am trying to make and rig a model in Blender. My model has to be symmetrical and have symmetrical weightpaint.
Unfortunately, my weightpaint is not symmetrical at least in some regions. I would like to draw your attention to the lips of my chatacter, especially two top corner bendy bones. As you can see on the pictures below, weights for corresponding bones are not symmetrical
Left side:


Right side:

Left side works fine for me, so I would like to make the right side to match the left side

What I have tried:
I tried deleting the “wrong” weightpaint vertex group for the right side
(this one)

And then duplicating the “correct” left-side vertex group
(like this)


And after that renaming it to “DEF-BB-Lip_Top.001.R” (so that it matches the deleted vertex group name) and mirroring the new right-side vertex group.
I use just “Mirror vertex group” because it ends up with a message “11477 vertices mirrored, 12 failed” (instead of 13147 vertices mirrored, 1312 failed with “Mirror vertex group (topology)”).
After this right side gets “correct” weightpainting. At least I think so
(like this, just like it was for left side at the beginning)

But now left side doesn’t have any weightpaint for the corresponding left bone at all

I believe that the fact that not all vertices are mirrored is bad. But I don’t know how to fix it either. If I go to edit mode, than to mesh and select “snap to symmetry” I get a message “11854 already symmetrical, 22 pair mirrored”. So since there are no failures, the mesh itself is probably not that bad (I hope).

But even after this mesh snapping-to-symmetry I get the same problem with weights and the same messages with “12 pair failed”

I did all that with “auto normalize” turned off and then I did it all over again with “auto normalize” turned on, but there was no difference
.

Is there any way to fix my issue and to make weightpaint from the left side get mirrored to the right side without wiping left-side weights out?

I am still new to Blender and I will be greatful for any help

here us a link to my file https://cloud.mail.ru/public/MWL8/HxXFhxxfw

If I go into edit mode and use the “symmetrize” tool, some vertices actually change around the eye. I guess those must be the reason for the failures. Of course, doing this completely breaks the weights on half the model, so they would need to be completely redone.

I am not sure if this causes the weird weights though, this seems a bit far from the failed vertices I found.

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Hi! Thank you for the answer!
I really appreciate it

Actually, I found out that it was weightpainting that was causing the trouble. Weightpainting on the left side had some minimal (like 0.001) weights in the right side as well.
I can be seen if you turn on “Zero weights”

Many thanks to @Pxy-Gnomes

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