Lookup fields with multiple selection becomes grayed while creating a new record using quick edit mode.
Once the record exist, using the quick edit, there is no problem for modifying lookup fields with multiple selection.
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Sign up to join this communityLookup fields with multiple selection becomes grayed while creating a new record using quick edit mode.
Once the record exist, using the quick edit, there is no problem for modifying lookup fields with multiple selection.
Are you working on SharePoint online?
I can reproduce your issue in SharePoint online modern experience.
Lookup fields with multiple selection works fine while creating a new record using quick edit mode in classic experience.