Hi, let me add some context here.
When an NTP source is added using config.json (the configizer tool in your case) they are added to the running chrony using chronyc, as seen in https://github.com/balena-os/meta-balena/blob/87dacf84d83d5bf6e999847da9e86d9026dde73b/meta-balena-common/recipes-connectivity/resin-ntp-config/resin-ntp-config/resin-ntp-config#L19, that is:
/usr/bin/chronyc add server $ntp minpoll 14 maxpoll 14 || true
/usr/bin/chronyc burst 4/10 "$ntp" || true
This happens at every boot, but no change is recorded to /etc/chrony.conf, which is stored in a read-only filesystem.
This works fine when the NTP address or hostname used is an NTP source. When it is an NTP pool that can be resolved to a random NTP source, the NTP source is added fine, but the burst setting in the second line above fails with the 503 as it needs a valid NTP source. So, the NTP source will work but it will take longer to synchronize as the burst setting did not apply.
We are discussing mentioning this corner case in the documentation.