A modern car has >100 ECUs, blink is distributed to a few ECUs, think: front body, rear body, left door, right door plus instrument cluster and maybe a trailer control ECU. It does not look premium if they do not blink in phase so you send CAN command for blinking. (A separate cable introduce cost, weight and faults due to corrosion that are tricky to diagnose, in total you want to avoid that.) If something disturb CAN or an ECU reboots you would get the described behavior. But it is not obvious (at least not to me) that it must be a software error - could easily be hardware designed with too small margins. (I have not worked with this issue at Volvo and do not know the actual cause of the problem.)