This typically happens, when you check **✓** *Filter Add-ons* in the *Tool* tab.

Filter Addons allows disabling certain addons under certain workspaces. Since Cycles is integrated into Blender through an addon that means it gets disabled as well.

Otherwise check to see if the addon is globally enabled in *Preferences > Addons*.

[![enter image description here][1]][1]


  [1]: https://i.sstatic.net/hK5SW.gif