“Full of grace” is literally “pleres charitos” and that wording is used in reference to St. Stephen (Acts 6:8). And, technically, anyone who was recently baptized or received the sacrament of confession is pleres charitos.
In Luke 1:28 the word that the angel uses is kecharitomene. So its not literally “full of grace” but its root word is the Greek verb “to give grace” (charitoo). The word is the past perfect tense, thus it means that the action of giving grace has already occurred. It was not something that was about to happen to her, but something that has already been accomplished. The word was also used as a title. The angel did not say, “Hail Mary you are kecharitomene,” but rather, “Hail kecharitomene.” Therefore the word is not simply an action but an identity.
The words and context used for Stephen and Mary are different and thus cannot be compared on a one to one basis.
