20amp afci breaker with gfci outlet tripping under 15amp load, but not lower loads
AFCI breakers are horrible with motor loads and even heavily loaded circuits with dimmers or ballast, to the point my state exempts circuits that have problems.

Why are they not able to tell the difference? Your miter saw creates tons of little arcs each time the brushes change from 1 commutator pole to the next, on top of pulling 3-5x the current for startup. This looks like arcs that could cause a problem. Dimmers and ballast change the wave shape causing harmonics that if a loaded circuit (well below the 80% standard) they will trip because they cannot tell the difference between the harmonics and an arc.

Using current code, you need GFCIs in the garage, not AFCIs, and if you want to cut with anything that has brush-based based motors or variable speed it will need to be less than 50&amp; of the breaker value on some of the mfg info I have read.