Not sure if this is a bug or just not intentional. After completing a mission regarding the tyranids and subsequently getting 3 new fleets spawned in as a reward one of them spawned in a not yet reachable or visible sector on the campaign map (it tried to spawn in the Nemesis Tessera sector but i had 3 fleets there already and instead spawned in the Belial sector). Now the fleet is stuck there as not even the inter system connections are active yet. I attached screenshots depicting the fleet in the Belial sector and the lack of the sector connection to Nemesis Tessera or even the lack of existence of the Belial sector. The other two connections to Nemesis Tessera are Medusa and Belis Corona).
just to complete the information,
I encountered the bug too and reported it as you did: https://forums.focus-home.com/topic/36063/imperium-campaign-reinforcement-after-the-invasion-part-1-problem-contain-spoiler
The bug happens right after completing "The invasion part 1" after you defend your 3 system from the tryanid fleet.
The next mission is "Iron Teeth" If I remember right !
In my case, I was able to move the fleet to tessera by clicking the warp (Arrow button) to tessera.
And zooming on tessera, selecting the fleet from the menu, and moving it directly in tessera system.