Looking at the traces, I can see some differences starting with the port setup: the ACM version doesn't handle some of the RS485 setup ioctls (specifically, two TIOCGRS485 ioctls on the RS232 case that succeed vs. one attempt that fails on the ACM case). Past that, read()s from the devices differ wildly (single-byte reads on the RS232 case, many bytes on the ACM case). As to the actual reason these are being made and how the code underneath handles those differences, I don't know since I don't have access to that code.
