Hello @AbaddonDrums and welcome to the forum 
This problem has been noted before and is also mentioned in this topic:
The vertical / horizontal mirroring tool in krita is not accurate.
There was a formal bug report: 427831 – Mirroring tool allows for subpixel placement without supporting it which is marked as Resolved: Fixed.
From what you say, it may not be fully fixed in some situations.
The problem you have may be caused by sub-pixel placement of the mirror line or by the characteristics of the brush preset that you use, or a combination of both.
Also see this topic:
Issues with multibrush in vertical mirror mode
Can you study the topics and the bug report to see if you can get any more information from them about this?
So you’re using a ‘pixelart’ brush that has no ‘leakage’ outside of any pixel?