This article describes about why all the Junos versions are not available in MIST UI.

The MIST suggested version for MIST-deployed devices is different from Junos suggested version and can be located in the UI, and it is recommended to use this version as it is tested specifically for MIST deployment.

For steps on how to find the suggested version, refer to [MIST] MIST Suggested Junos Releases for SRX Series Devices.