You can see the brush thickness in the orthogonal slices to verify if you indeed paint only in a single slice.
It may be confusing to edit a segmentation that has higher resolution than the master volume (because arrow keys on keyboard jump by one slice thickness distance but you may not be centered on the slice; slide offset slider above the slice viewer jumps a small amount, which is typically somewhat smaller than the slice thickness, so it may put you somewhere off the center of the slice). It may be simpler to just resample the master volume using Crop&resample module before you start segmenting.