@joseph I'm having the issue right now. And I think I've found a pattern.
It seems to be nearly each time an app updates. And then it breaks my PTR6 record in Cloudron only.
Works fine with MxToolbox (but it switched back to the second nameserver as the last time when it wasn't working on cloudron side for the PTR6 record and the first NS reported the PTR4 record => I still don't think there may be a link but just writing it here in case it could be a lead ) .
Below you will find the output of the following command :
host -t PTR <ip6> 127.0.0.150
Edit : As a workaround I've rebooted my server and now it works again in Cloudron and MxToolBox. But we know it's just a temporary fix... (In mx toolbox the PTR6 is reported by the first NS now. And the PTR4 by the second NS).