Sometimes Mari seems to have small issues with the texture bleeding.
I just realized that sometimes the bleeding doesn't happen at all. If you find yourself with this problem, the best solution is probably to force Mari to do the texture bleeding.
Only 2 steps are needed.
- Click on "Toggles on/off whole patch project.
- Now select the patch or patches, right click on top and select "Bleed patch edges".
- This should be enough to fix pretty much all the texture bleeding issues inside Mari.