Hello Olivia,
The theme has multiple tour types, some allows users to select their preffered start date of the tour (Simple tours) which doesnt have any specific date range when the tour takes place. Hence there wont be any point in showing them in date filter as they have no date range.
The other tour types, which takes place from / to specific dates, they will appear in date filter. As for disappear, indeed they will not be shown in frontend, because if you have a tour starting on 7th of Aug 2020, there will be no point in shoing them. Hence they are automatically hidden once that date is reached. This is a feature added as per multiple requests.
As for the woocommerce bookings plugin, this is third party plugin. Having said that, its main feature is to allow users to book any available date, hence I am not sure why/how this will really work with the date filter.
Perhaps, your use case may be different and date filter is not required at all? If you would want to remove the date filter, you can do so easily by removing the line for date_range from your page block.
Please feel free to ask if i can help / clarify anything further.
Thank you