This bug is rarity for me. Happened one time during a Legendary Contract for Yarrow. Just happened again today around 8:30 am (PST).
Occured after a squad wipe against the Swarm Tyrant in the Tyrant Mine Stronghold at Grandmaster 2 difficulty on Xbox One S. The rest of my squad went down a second time and when I went over to revive them there was no button prompt.
When this occurs I usually just wait for them to respawn and if they go down again it fixes it self. No video footage this time around as I wasn't recording.