When you enter time signatures, Dorico only rearranges the existing musical time into the new bars, up until the next time signature (or manual barline).
If the last bar is incomplete, it is not padded out, unless you have Insert Mode turned on. Rather it is treated as a pickup bar.
Here you created a 3/4 with only room for a 3/8. So Dorico say that’s fine, here’s a 3/4,1.5 pickup bar followed by a different time signature!