Out of space problem is probably from left overs from old configuration, logs, graphing, possible file system corruption. Export full configuration (not backup), net-install and restore exported configuration should correct the issue.
Differences between free space on identical configured routers can be due to logging, graphing, possible left overs from file system issues or upgrades.
Most of these issues I have seen from routers that have been upgraded from ROS 6 to 7, and have not been net-installed to completely refresh the file system and restore the config export (not from a backup). On routers with small storage I also never keep any extra files on them, always remove any backups/exports for safe storage.
I agree with your statement, so in the past, I also performed netinstall on the problematic devices, but found that the issue still persisted. To be honest, I'm not sure which configuration caused this problem.
The official ticket has identified the problem and confirmed that there is a leak, but there is currently no specific timeline for the fix. They also did not inform me which of my settings caused the leak. It seems that I'm the only one who has experienced this issue.
By "experienced", I mean on a fresh netinstall of RouterOS 7 on hAP ac2.
The hAP ac2 device has a small 16MB HDD, which is too small to use RouterOS 7.x.
Using the same code on RouterOS 6.x on the same device does not result in any leakage issues, indicating that RouterOS 7.x still has a long way to go.
I believe that MikroTik should issue a statement similar to the one previously issued for CRS3xx devices until the leakage issue is resolved.
RouterOS 7.x is not recommended on the hAP ac2 device.