Some inks do this much worse than others. Two option: don't stack hot (use a fan on the outfeed, shake the shirt for a sec to cool it, etc), or switch to an ink that doesnt ghost. I believe the synergy poly is advertised as non-ghosting, though that ink has waaaaay too much puff in it for my liking. My exit temp with that ink was 320, though i believe 300 is the advertised cure temp, but in our short-ish electric dryer getting a full cure at a lower exit temp wasn't possible.