The notehead size of the first sixteenth note also jumps back to 100% when adding the tie. Dorico sees a chain of tied notes as one single note and doesn’t allow individual notes in a tie chain to have e.g. separate notehead properties (there was a topic recently about an X notehead on an eighth note changing when a tie was applied). Because the sixteenths are all beamed together, the entire beam is affected as well.
I’m not at my Dorico machine to check, but the solution to that other topic might also be helpful in your case (i.e. either try if you can change the properties in Engrave mode, or work with different voices).