The reason is in Basic Team Mode, you only have one player per team. This doesn’t mix in a cumulative way with everyone playing in Advanced Team Mode. You’ll have some players that have scores from just Round 2 and then you’ll have the Basic Team Mode leaders who have scores from all three rounds. It won’t work out and be fair.
If you did want to this scenario and you’re not worried about Multi-Game Scoring and just want per-round results, then it could work. Round 1 and Round 2 would work fine. It would be the Round 3 that would be tricky as Round 2 goes from everyone playing, which is how Advanced Team Mode works, to going back to only one team leader per team playing. So as long as you can communicate that not everyone is playing, then going to Round 3 would work.
It would look something like this:
Round 1: Basic Team Mode - only one team leader plays on their phone, they enter a team name and enter answers on behalf of their team (e.g. a table of people all on a team but only one person is actually playing). After this round, then Next Round to Round 2.
Round 2: Advanced Team Mode - now everyone is invited to join and play on their phones and choose from the list of predefined teams. So now all people are playing at the table. NOTE: The players who played in Round 1 would have their nickname already set as the Basic Team from Round 1. So after doing the Next Round, you may want to do a full reset in order to remove all the players from Round 1 so they have to enter their actual nickname again and then choose a team. Otherwise it will retain their nickname from Round 1 which is the Basic Team name they entered. After this round you would Next Round to Round 3.
Round 3: Basic Team Mode - in this round it goes back to only one team layer playing on behalf of the team. And like Round 2, you’d have to do a full reset after the Next Round or all the players from Round 2 that are the team leaders in Round 3 would retain their nickname/first name as their Basic Team Name (these fields are shared by the two modes).
Overall, this doesn’t sound like a great workflow, so we don’t recommened it. Either do all Basic Team Mode or all Advanced Team Mode. Or if you have to do both, don’t switch back to Basic Team Mode after going to Advanced Team Mode.