I asked this question regarding the gate spell targeting someone using astral projection and the results were inconclusive. One thing that I realized however, is that gate can only summon someone on a different plane as the caster.
When you cast this spell, you can speak the name of a specific creature (a pseudonym, title, or nickname doesn't work). If that creature is on a plane other than the one you are on, the portal opens in the named creature's immediate vicinity and draws the creature through it to the nearest unoccupied space on your side of the portal.
This makes the original question even more complicated, but also poses a question for other spells such as sending which might fail if a target is on a different plane than you:
if the target is on a different plane than you, there is a 5 percent chance that the message doesn't arrive.
And also scrying which can only target a creature on the same plane:
You can see and hear a particular creature you choose that is on the same plane of existence as you.
There may be other such spells, so I'm asking in general if a caster is attempting to target someone whose physical body is on the same plane as them but under the effects of astral projection, will that target count as on a different plane or on the same plane?