Tied notes do not playback decrescendo

Hi @jazzcafe , and welcome to the Forum!

As you can read in this guidelines, it would be faster to diagnose the problem, if you could share the Dorico Project.

Probably you need to define an end dynamic, so that Dorico knows how soft it should sound at the end of hairpin…(double click on hairpin, write pp in the popover, and press enter). What was the last dynamic before the hairpin?
Also there are settings for the Dynamic Curve (in Playback options, but some values are overridden in the expression maps): search the Forum for dynamic Curve and you will find some discussions about it.

Which Playback template are you using?