Not sure if it's a bug or a design limitation, but Server Actions seem to only be triggered by a button signal, not by workflow signal.

Try changing your view xml: change the button name from a method name (which triggers workflow signal) to the signal itself. E.g. in Sale Order, that would be <button name='order_confirm'> instead of <button name='action_confirm_order'>.

Or add another dummy node to the workflow with an outgoing transition requiring a button to be pressed and set a button signal as the button's name (not an object method which triggers the workflow signal).

More here.