What happens if I add the domain to Cloundron so I can use it to setup sub-domains for NodeBB and Ghost?
When you add a domain:
To validate credentials, the code will add a cloudrontestdns A record and remove this immediately.
To configure the server to send emails:
If there is an existing SPF record, it is edited to have a:my.my.domain.com . The SPF record essentially whitelists which servers can send email on behalf of the domain. This is NOT destructive, your existing SPF record is only amended.
A DKIM record is added under <random>._domainkey.domain.com. This is used to sign/verify emails by mail servers. Because of the 'random' part, it won't conflict with any of your existing DKIM keys
If there is no existing DMARC record, we create one with the value "v=DMARC1; p=reject; pct=100". This essentially is a strict DMARC policy not allowing anyone outside the SPF record to send emails on behalf of the domain.
Other than that, it is what @BrutalBirdie said. The bare/root domain is untouched. Cloudron only adds entries when you install apps. Also, if you had a root domain DNS entry already, it will warn you that it's in use, if you try to install an app on the root domain (it does this check for any subdomain).
@LoudLemur looks like the DNS record was pointing to a Cloudron which didn't have any app installed on that (sub)domain. If a Cloudron server as such just gets deleted, then the DNS records are not automatically cleaned up, since there is nothing running anymore to do that.