I'm speculating, but I'd suspect that your traffic connects and initially passes through your guard node while traffic passing through your relay is probably going to other nodes in the network (since guard nodes are used for initial connections and as a relay you're already the middle node).
In this case it wouldn't help conceal anything extra from your guard node.
For those who don't know, guard nodes are the nodes you initially connect to. In Tor once established, your first hop is always to the same node - this is because it's assumed some nodes are bad actors and if the first node is randomly selected each time your chance of eventually connecting to a node trying to collect information is high and partial compromise isn't much better than fully compromised.
By selecting and using one guard node for your initial connection it's either a bad actor or not, but if it isn't then you're good to go from then on.
When you run a relay if your relay is fast, stable and online for a while (60 days I think) the Tor network will automatically turn your relay into a guard.
In this case it wouldn't help conceal anything extra from your guard node.
For those who don't know, guard nodes are the nodes you initially connect to. In Tor once established, your first hop is always to the same node - this is because it's assumed some nodes are bad actors and if the first node is randomly selected each time your chance of eventually connecting to a node trying to collect information is high and partial compromise isn't much better than fully compromised.
By selecting and using one guard node for your initial connection it's either a bad actor or not, but if it isn't then you're good to go from then on.
When you run a relay if your relay is fast, stable and online for a while (60 days I think) the Tor network will automatically turn your relay into a guard.