You are doing it right but in this case the words acts as a noun/activity (usual in infinitives). Disambiguation processes its double meaning and removes the verb form in falar.
The rule is solid, but disambiguation will act on every case that you have a preposition before the verb. Disambiguation can be improved to cover this case, but this is still ahead of the level of precision I am focusing for the moment.
I believe you can make it solid using a workaround like this;
<token regexp='yes' case_sensitive='yes'>.*[aeiou]r</token>
This will match all infinites without caring for the disambiguation. It will fail with the infinitives that require disambiguation, but you can add exceptions to the problematic ones.