Hi @annakjacobs,
This is the expected behaviour: because Dorico considers tied note as a single note whose value is the sum of all current tied notes. Like in a DAW:
ex.1
Before inserting the dot you have a half note (visualised as two tied quarter notes). Inserting the dot, you are giving a dot to this half note (and not only to the last quarter, as in other software). So the resulting tie chain length is 3 quarter notes long (which is the quarter note tied to the half note that you describe)
So instead of clicking on the dot, you can just prolong the half note tie chain, and, with the rhythmic grid set to eight note, using the prolonging shortcut: Shift+Option(Alt)+right arrow once.
ex.2
In your example you could write all without dots, and then add them (with the moving shortcut) all at once. Dorico will handle the eight note automatically:
ex.3
(Or another method would be to write the separate notes, first [a quarter, and a dotted quarter] and tie then afterwards)
For more details, see the Manual: