As @Slowwithouttwitch said, the logic is generally to give you the number of workouts per week as requested in the Plan Builder setup flow while maintaining a similar volume to what you’ve recently been doing.
Adding the volume to a different workout may trigger Red Light Green Light warnings (it seems to have done so in your case) if it “upsets” the TSS balance of your overall week. As an example, yellow/red days often start to pop up after multiple higher-stress days in a row, which I’m guessing is what’s happening in this instance if you’re getting those warnings after adding the extra volume to Sunday.
If you have a rest day on Monday, though, I wouldn’t worry too much about the warning – yellow/red days will typically clear after a day totally off.
Ultimately, you know your schedule best – so if skipping the 30-minute midweek session and adding that time to another ride in the week works better for you, then go for it! After a few weeks of training with that schedule (or similar), Adaptive Training should pick up on what you’re capable of doing and adjust your RLGL warnings accordingly.
Hope that helps clear things up – feel free to let me know if you have any other questions!