Emily Coutts (Detmer) and Oyin Oladego (Owo) are never listed with the main cast characters. They and others like Patrick Kwok-Choon (Gen Rhys) are credited in the ‘also starring’ list in the post credits not in the main titles.
No reason not to believe Emily won’t be back. Oyin was in production as the lead in an independent feature last year so there may have been a scheduling issue as there was for Ronnie Rowe Jr (Bryce) when he was in production for his lead role in BET+ Series ‘The Porter’ during production of Discovery season 4.
Most of these Canadian actors filling in the bridge crew work on other Canadian projects which seem to be more crucial for advancing their careers even if working on Discovery was a regular income.