Third time making this pattern - notes from previous versions. Working on them while traveling and between other projects.

On thumb gusset: M1L, knit, M1R for increases.

These run large - make S for snug mitts but use M number for thumb gusset.

Pick up 2 stitches rather than one at the thumb intersection.

Try this (from another project):
When picking up stitches for thumb, pick up this first stitch with M1L (insert Left needle UNDER the bar from front to back). Pick up the last stitch with M1R (insert left needle from back to front). This helps eliminate holes at the sides of the thumb.