This issue was addressed in release 11 of the ruleset. See RS2.071 here:
https://docs.google.com/spreadsheets...it?usp=sharing
The DEX modifier is still shown on the combat tab in the AC section, but the AC total should be adjusted for the armor stats if the check box in the "Armor" section of the Inventory tab is selected.
If this is still not working, make sure you're running release 11 of the ruleset (shown in the chat message on loading) and also refer to the "Notes" entry for RS2.071 in the sheet linked above.