Thank you, but I don’t have the EAP or FSD. That may or may not make a difference though. I have the Basic Autopilot. So I’m referring to Autosteer with TACC. Currently, when I turn on the signal, auto steer stays on until I apply input to the steering wheel, at which point it deactivates for me to make my lane change. What I have noticed though is that it only alerts me of blind spot traffic with the chime and the car next to me being animated in red when I am not on Autosteer. If I have Autosteer on and engaged, and I signal left or right with someone in my blind spot, that car animation for the car in my blind spot remains Grey and I get no visual notification of the blind spot warning.
This sounds like a bug.
Basic Autopilot (excluding the original AP1 installed to early Model S cars, but that is not relevant here) does not perform lane changes.
The lane is probably checked only when you initiate the signal and Autosteer is off.
If Autosteer is on, it ignores the use of the signal and takes no action, because BA doesn't do lane changes.
However, when you tug the wheel to move lanes, it should - but doesn't - check the adjacent lane for obstacles because when you initiated the signal, Autosteer was enabled.
It sounds like a good test would be:
While using Autosteer, signal to change lanes when the adjacent lane is not clear.
Tug the wheel to deactivate Autosteer (don't actually depart your lane.)
Cancel your initial signal.
Signal again (with Autosteer still deactivated.)
If the lane is still not clear, you should receive a warning.
IMO, the adjacent lane should be checked BOTH upon initial signal activation AND at deactivation of Autosteer if the signal is still on.
Also, you said "new Model 3." Does it have the USS? This might completely change how the car detects whether the adjacent lane is clear or there's another vehicle in the way.