Android 7.x+ came with new features, like Battery Optimization, Standby Mode, Power Saver Mode, and many others.
These features set applications to a complete sleep so they cannot work on your device background if you close the app.
Basically, these optimization apps are killing Appointfix services and the app can not wake up to send the scheduled reminders in time.
Here are the steps to disable this function for Appointfix.
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Hmm I dont have "battery optimization" setting when I go into that menu. The only option I get on the Moto-G is "battery saver". Guess that option to toggle the setting didn't come until later android versions