This issue is related to the current absence of a way to add an alternative host to a single occurrence of a recurring meeting.

If any of the recurring meetings' instance has a specified date, users need to choose Edit All to add an alternative host, and the Edit All option is not available when recurring meetings are created with no-fixed-time. This results in the blank Alternative Host field when editing the meeting.

The vendor has a feature request open with their development team to make it possible to add an alternative host to a single occurrence of a recurring meeting. Zoom has not provided an estimated timing for when this feature change will be implemented.