Older unions after separating glitch and their colours change

[OndrejJePan]

After separating the doors and again unioning them with same negates they somehow go to darker colour on the position of part where negates are used.

This is how door frame looks before separating:

After unioning them again:

(Notice the darker parts of the inside)

This bug never happend to me on older roblox studio versions, I often remaked older unions to fit with new part of building but now this happens.

Note: I tried to to change negates to colours that are the same as I want the whole union to be but that also did not help.

Anyone got idea how to fix it?

[OndrejJePan]

I just solved it by going over it with another method.

If anyone wants to fix it you can follow this:
You can go over it by exporting it and then publishing it back to your place by placing it as mesh to a meshpart.

Bare in mind that after that you cannot separate it or anything similar that is only feature for unions!

[RMofSBI]

Theres a better faster easier way:

Once you have your new union just click on UsePartColor:

You can still edit the union as you normally would.