My logs are also flooded with this and from what I gathered, this might not be directly related to DAWN but seems to be a bug in the cfg80211 kernel module (driver) related to 802.11r. Some forum threads suggest that it's a regression that happened after OpenWRT 19. Unfortunately, I can't downgrade to verify because my devices (TOTOLINK X5000R) only got support since OpenWRT 21. There's a patch online (not upstream, unfortunately) and rumor has it that it could mitigate the bug.
This is why I'm trying to figure out building OpenWRT from source. I haven't succeeded doing that but here's the details: Daemon.err hostapd: nl80211: kernel reports: key addition failed - is this a problem? - #45 by fodiator.
At least, that's my initial impression, at least. I've been googling so much that I don't remember how I came up with the connection to that forum thread, though. Maybe somebody like @PolynomialDivision would be able to give better pointers...
Note that https://github.com/openwrt/openwrt/issues/7907 (ex. https://bugs.openwrt.org/index.php?do=details&task_id=3159) suggests that it is recommended to set FT Protocol to FT over the air and Reassociation Deadline to 20000, specifically referencing this sleep mode case (though, mentioning iOS, not Android).