This article explains the issue where MIST configuration push takes time to reflect on the SRX.

When you try to save any configuration on MIST portal, it takes more then 30 minutes to reflect on SRX.

This issue could raise if the SRX is running on high resource utilisation like high RE CPU and you try to push the configuration from the MIST portal at the same time. This could result in over loading the CPU and the firewall can disconnect from the MIST portal. It is recommended to wait for the CPU to get stable before pushing the configuration.