If a user adds custom properties to one or multiple parts, that property should have its own column in the BOM export.
I have added a property "JLCPCB#" to multiple parts in a project. Instead of being separated, it is in the column "Other Properties" where it is only separated by commas rather than columns.
To replicate: Add multiple unique properties to a part in a project and assign them a value. In the exported BOM of that project, you will find all of the properties get added into a single column.