I have a ramips mt7620a -based device that is losing it's sysupgrade.tgz config when upgraded from LEDE 17.01 series to 18.06.1. There seems to be some incompatibility when doing the 'zcat sysupgrade.bin | mtd write -j /tmp/sysupgrade.tgz - firmware', where it integrates the sysupgrade.tgz into the jffs2 filesystem.
This is a custom device based on the IAR-142-G: https://oringnet.com/en-global/products/detail/P0000000512/91/?IAR-142-4G_US
There is plenty of free flash space (writing to a 24MB partition, ~10MB free.) This target is using a single 24MB firmware partition, having a uImage header with a kernel + rootfs, and the root filesystem is split squashfs + jffs2.
I've tried using the new mtd binary to write the image (from the old kernel/firmware), but that had no effect. I've looked at the diffs between mtd, mount_root, and kernel jffs2, but I don't see significant changes there.
Some other data points: same codebase for x86_64 and mvebu targets with ext4 upgrades/downgrades OK (they stash sysupgrade.tgz using a different mechanism). Also this ramips jffs2 downgrade from 18.06 loses its config.
Any tips on where I should be looking -- for what has changed between versions?