Hi @Rachel_Fuller , welcome to the forum 
I would consider exploring the option to switch from using a multi-select field to a single-select field. If that is not possible, try creating a single rule for each possibility but that would probably still not work for you, unless the tasks are created via a form from which all multi-select options would be applied upon creation, once and not added gradually. Unfortunately the rules are missing the ‘if added to the multi-select’ or a ‘contains only’ option, then…
You could vote for this here and add your voice to that thread.