FP < F : What's the trick?

It is there, but quite subtle indeed. The problem is you don't have a true dynamic marking after the fp - the "f" is just text attached to the hairpin. So instead, MsueScore is only crescendoing to the "mp" in the next measure.

The hairpins with text attached probably shouldn't be there in the palette, all they do is cause disappointment in cases like this. They were added before we handled the layout of hairpins followed by true dynamics more correctly, and no one thought to remove the hairpin-with-text workaround that had temporarily been added to the palette. On the other hand, it can occasionally be useful in cases like this where the end of the hairpin isn't on an actual note, and the next note is a different dynamic.

Anyhow, two solutions:

1) keep the hairpin-with-text as is, but edit the velocity change in the Inspector to be more dramatic (the default 0 means look at the next dynamic, which is only mp). mp is normally 64, f 96, so a change of 32 would be exactly right as far as that goes

or

2) delete the text from the hairpin in the Inspector, then add an "f" to the rest. You can then position it further left if you like 0 and you'll see the hairpin shorten itself as needed (which is what we weren't doing when the hairpin-with-end-text hack was added).

Either way, I would also recommend changing the playback of the hairpin from the default linear curve to "ease in". This to me does a better job of mimicking the specific way horn players to crescendos after an fp, where they typically hold the quieter volume for longer than otherwise and then crescendo more dramatically towards the end.