Sure I am running BIND.
'dns: ' I cannot remember where I got that... probably followed some guide and it wrongly ended up in my definition.
2 separate lines... OK
See, I am using virtualmin:
virtualmin modify-template --name "Default Settings" --setting dns --value '${DOM}. IN CAA 0 issue "letsencrypt.org" ${DOM}. IN CAA 0 issuewild ";"'
The value is a string, I guess I have to echo the string with a new line separating the 2 lines.
Thanks to all