I had the same problem with Debian 10 also.
In my findings it seemed that the "commented out" network config that XO ships with was throwing it off.
The workaround in my case was...
- Insert my cloud-config as per usual.
- Remove everything from the network config box (if I didn't want cloud-config to set this up for me).
- Insert a proper network config if I wanted network setup also.
Hope that helps