Something changed in Forms lookups to where the instead of showing the Drop Down manually assigned values until the lookup is complete, the values are populated in real time while they are pulled from the database.

This loading symbol also shows to let you know values are still coming in.

However if you select a value while this symbol is showing, from the partially loaded values, when the lookup is complete, your selection will be removed.

Is there any way to prevent the user from selecting a value while the lookup is running?