Sysupgrade fails if console is set to ttyS1

I am working on MT7620A based device and ported OpenWRT and everything works fine except sysupgrade. I want to use it's console port ttyS0 for some purpose so I set following in the DTS.

	chosen {
		bootargs = "console=ttyS1,115200";

If I set that, I see that debug log is still printed on ttyS0 but console is disabled on ttyS0. However, it also has an undesirable side effect that sysupgrade also fails. When I try to upgrade OpenWRT, device simply reboots without giving any kind of warning. How do I solve this?