As explained in the URL below, the "join before host" option is not available if the start time is within 15 minutes of the current time.
I'm not sure why this is the case and it causes confusion for users who have this option enabled by default.
This option is only greyed out during the initial scheduling phase. If I immediately edit the meeting, leaving the start time as is, the option "join before host" is then available. So why does this needs to be greyed out during the initial scheduling?