When doing sysupgrade, we switch to the second partition with a separate rootfs_data: Adding OpenWrt support for Linksys MX5300 - #25 by lytr
The next sysupgrade will allow you to return to the current rootfs_data partition.
I'm thinking about adding an option that would allow sysupgrade on the same partition: Adding OpenWrt support for Linksys MX5300 - #35 by lytr