I just got confirmation from the iodé lead dev that if the bootloader is unlocked then OTA updates or adb sideload will fail to apply the update (as you see) due to corruption with slot b (so it can’t do the a/b slot handoff). We understand this to a “bug” in the Mediatek firmware that has not (yet) been addressed by them.
To resolve this, you need to use SP Flash Tool as guided here but use the “Firmware Upgrade” option. This is the only thing that the iodé lead dev has found will correctly reset the device to then work with OTA updates. You may be able to uncheck userdata so that you won‘t lose all your data, but likely Firmware Ugrade will reset everything.
Note that SP Flash Tools (Windows and Linux) can be downloaded here:
SP Flash for Linux: https://spflashtools.com/
You will need a full image to use with SP Flash (the OTA .zip is not enough as it doesn’t have the other partition info and important download_agent/flash.xml needed by SP Flash Tools). Here is a link to the latest full image hosted on our gitlab: brax3/iode-6.6-20250803-brax3-spflash.zip · master · ota / release · GitLab
EDIT: I originally explained wrongly that it was the unlocked bootloader itself that was the reason OTA updates couldn’t be applied. I corrected this above to explain that it is the unlocking of the bootloader that seemingly corrupts slot b from being able to be used, which is why OTA updates will fail.