I'm guessing it's kinda like someone using Fire abilities without burning themselves or ice abilities without freezing, the common trope of not hurting yourself with your own attacks, which not always are given an in-story explanation that the user has a natural resistance or such (sometimes it's just a normal person that had just discovered their powers). So I guess unless there are explicit feats of resistance or statements that users resists one must assume they don't?
IIRC there are cases of Time Stoppers not being resistant to it, for example, in the Unordinary Webcomic, Seraphina and her mother both have time controlling abilities, and at one point they use them on each other, but Seraphina wins this duel because she can trigger them faster. The interesting point is that none seemed to actually resist each other's powers, and Seraphina simply won because she stopped her mother from triggering hers.