Summoning a Marker entity at some position will prevent any armor stands from being placed in that position.
REPLICATE BUG:

try to place an armor stand in the block where the following command is executed: /execute align xyz run summon marker ~.5 ~.5 ~.5

I fear that because the client can't place an armor stand in that block, this means that the client is aware of the marker in that space.

The only reason that a client would be aware of a marker in the space is that the server is sending marker position packets to the client which shouldn't happen if I'm correct.