If it does it again and you are at a slow enough speed try doing a manual pull down of the shifter without getting off the throttle.

If it instantly starts to engage my first guess is it is stuck in high gear. May feel like neutral but most stock converters stall around 1700-2000 so if it’s in 3rd you aren’t against the converter. The true test for a gov issue is to install a 0-80 lb. gauge and see what’s happening when you get this issue. Pressure should track close to vehicle speed, 5 psi tab 5 mph, etc.