did you noticed the notification during reinstall??
[ INFO ] DietPi-Software | AdGuard Home executable "/mnt/dietpi_userdata/adguardhome/AdGuardHome" already exists. Download and install steps will be skipped.
- If you want to update AdGuard Home, please use the internal updater from web UI.
- If you need to reinstall (e.g. broken instance), please manually remove the executable "/mnt/dietpi_userdata/adguardhome/AdGuardHome" and rerun "dietpi-software (re)install 126".
therefore, try following
rm /mnt/dietpi_userdata/adguardhome/AdGuardHome
dietpi-software reinstall 126