This really depends on what you mean by segment.
Do you mean an attribute that is set in the SDK that you maintain ownership of? If you:
• set an attribute in the SDK whenever that event happens near them
• target your experiment only if they have that attribute
Then your experiment sample will only be made of people for who:
• You set that attribute in your app based on whatever criterion you wanted, who then, while still having that attribute, hit the feature flag in the code path
These people will stay in the experiment for data purposes even if their attribute value changes (e.g. you turn it off in the app because it has been X time since the event you care about).
That is the desired behavior from an analysis perspective, but to make sure you're measuring the treatment effect, you might want to design your metrics to use "conversion windows" so that you only measure behavior in the X hours after experiment exposure.