This content creator (link to youtube video (German Language) - https://www.youtube.com/watch?v=wdy2TpqIPVk ) claims the same.
Short summary of the video: it’s a “feature” - the sim cycles the availability of different mission types. If you have many different mission types unlocked you’ll notice - e.g. Charter Private missions disappear and search/rescue-hoist missions appear. If you have only a limited amount of missions unlocked (or limited company options) then it seems that missions disappear because the alternatives are not yet unlocked and hence are invisible.