The smooth tool makes gap in mirrored object.
I would consider that a bug.
It looks like some of your vertices aren’t exactly aligned along center of the mirror axis, so the merging option isn’t affecting them. i.e. they are outside of the range for that. either increase the merge limit value or move the vertices to the middle.
If that still doesn’t help, then report back. TBF verified bugs should be reported to the tracker. You can do that by going to the help menu in blender and picking the
Report a Bug menu item.
I have had this issue with many sculpt brushes. It will offset any mirror modifier geometry and pulls them apart, even with a high merge limit.