You should have said this before: because it makes for an easier investigation for us. Does it still happen if you choose a "Restart Couatl" during the approach, and then select the Gate immediately after the restart is completed ?
I mentioned it later because I didn't explore this until after the original post. On my last four sessions, I always experience waiting for other services if I assign a gate inbound (2) and twice I assign gate after on the ground and do not experience the waiting state.
Next session I will restart couatl on approach and select gate before landing and report.
UPDATE: I restarted couatl an approach to KATL, below 250/10000 and within 25 nm, then couldn't bring up the Shift+Ctrl+F12 menu for about two minutes after restarting couatl. When able, I did select a gate while still on approach and airborne. On parking at the gate I called up the menu to operate the jetways and I got a waiting for other services status.
UPDATE: Another day and another session. As opposed to last session where I restarted couatl on approach then selected gate and encountered a waiting for services hangup, in this session I reverted to waiting until leaving the active after landing to request parking and all proceeded as expected. I am confirming a pattern, but why am I only one reporting this repeatable pattern?