I’m seeing this on Win10 Pro, and my friend also encounters it on the Mac version.
When you are playing in a Battlegrounds group with friends, if you get knocked out, the spectate button is conspicuously absent. In order to spectate, you have to leave the group, and then spectate normally. Normal spectating always works–it is only when you try to spectate from within the Battlegrounds party that it no longer works. This is 100% consistent.