Morgan Box on Freightliner Chassis

Has anyone had any experience with Morgan truck boxes with cracking in the vertical posts? I have 2 x 2020's with Freightliner chassis and 1 2019 with Freightliner Chassis and all three are cracking in the exact same place. These are 28' box trucks with heavy duty floors and liftgates. We ship corrugated boxes, so the loads are not that heavy. Cracking appears to be coming out at the rear tail lights (on the drivers side mostly,) but in the 2019 it's pretty severe on both sides at this point. We have back up camera's and rear warning sensors so the drivers are not smashing docks. It's also not 1 driver- it's at least 6-10 over the last 5 years. Any info or experience you have that may shed light on what's going on here? Normal wear and tear? Morgan defect? All three are leased. Thanks!