When I enter this tie chain and then make it 3-slash tremolo, I get 4-slash on the dotted half note. If I change to tremolo before I tie, then it is correct. To fix, I merely untied and retied.
The notation is logic and correct, a slash equal to a half-value of the musical figure.
For circumstances where you want to change the number of slashes on individual notes in tie chains, and if you have Dorico Pro: switch to Engrave mode where you can select each notehead individually and use the available property in the Properties panel.
Thank you everyone. I learn something new every day here.
I learn something new here most days 