If you use Thunderbird Lightning Calendar and create events using that plugin, by default, events in your calendar will be created with status "free".
This is a bug in the Lightning Calendar because newly created events should be created with the "busy" status by default.
Harmonizely checks if the events in your calendar are "busy". When the event is set as "free", on the scheduling page the date & time will be still available.
Solution: After adding an event via Thunderbird Lightning Calendar, edit it and change the status from "free" to "busy".