When companions' health bar is depleted, the companion will be in a downed-state and revive once combat has ended. However, on SuperNova difficulty, these deaths are permanent and is by-design.
Added October 31, 2019
Issue: Players are experiencing an issue where companions are dying without reason. This is due to a known issue where speaking to an NPC while another NPC is climbing a ladder, will cause the NPC on the ladder to fall to their death after the player exits conversation.
Workaround: To avoid this issue, please refrain from speaking to companions on the player's ship while another companion is using the ladder. We'll have this issue resolved in our next hotfix.