Out at first not a force play

[Marty Rogers]

I'll give this one a try (with help from Jaska/Roder).

A force out can occur when there is a consecutive runner. A runner is consecutive if, at the time of pitch, each lesser base is occupied. R1 is always consecutive because there is always a batter, R2 is with R1, R3 is with R2 AND R1. In these situations, the runners noted MUST leave their bases for the following runners advancing behind them.

The force out does not apply to the batter-runner because he is not consecutive (by definition he cannot be forced).
So, that's the explanation I could find.

Per situation #2: If BR misses 1B and defense appeals successfully, the run by R3 cannot score. The third out is the batter-runner before touching or passing 1B, or the batter-runner on an appeal for missing 1B, or when a batted ball is caught. 4.09a