here is my stab in the dark......
there are 2 basic functions in a trim, first is the cutting mechanism under the needle plate that actually cuts the thread on stitch finish.
2nd is the hook above the needle plate and behind the head that hooks the thread and tucks it into the felts of the thread hold bar reay for the next run of that color.
MY GUESS is on the puff sew when the machine gets to the end of a stitch run the machine slows down and because of the reduced speed and the DRAG the puff foam and tight stitching of the thread on puff work the needle is not coming up out of the sew as fast or smoothly as normal. the cutting mesh under the needle plate fires thinking the needle is already gone based on normal timing and not only cuts the thread but shears the needle as it is still partilly below the needle plate.
This might explain why your polos bed sheets etc are not bringing the same result. The needle lifts out at normal "drag" speed / timing if you will and the cutting mechanism misses it. The association of these two processes is very closely timed and the puff drag on the needle I suspect is the culprit.
Two things to try...
First go into your machine settings and switch the trim length option to the longest trim setting.
Second pull you needle plate the cutter is immediately below that . Inspect it are there and nicks in the cutting edge.
good luck
here are some links that may help
https://www.youtube.com/watch?v=d9FBBV_Z9jQhttps://www.youtube.com/watch?v=O3wI9ZCQQKkhttps://www.youtube.com/watch?v=Wmx7JuSukakmooseman (glad to be out of the embroidery business)