I think I'm having the same problem like described in [ SOLVED ] Getting error failed to sync jffs2 overlay where jeff posted that it's "bad timing handling the erasure mark on the tplink archer c7v5". What is different in my case is that the only jffs2 error in dmesg is the above mentioned "word at offset" thing. I do not get the "failed to sync overlay" message.
It can be reproduced scp uploading ca. 4 mbytes to the tp link archer c7v5's /root . Set sysupgrade.conf to persist /root . Upload openwrt-sysupgrade.bin image to /tmp and run sysupgrade -k via ssh. When the router reboots after flash (like @jeff described it as first reboot after flash in the topic) I had 28 failures and 3 successes trying the same procedure over and again. (This can be reproduced without using the offline installer scripts from my first post.) Failure means the jffs2 erase error message was there.