其实不是 an agreement 后置，而是时间状语 on Monday 前置了。其原因是 an agreement 后面有多个修饰语——in principle 和 with the Liberty Media-owned sport.

General Motors plans to enter Formula One with its Cadillac brand in 2026 after announcing on Monday an agreement in principle with the Liberty Media-owned sport.

= General Motors plans to enter Formula One with its Cadillac brand in 2026 after announcing an agreement in principle with the Liberty Media-owned sport on Monday.