When I try to edit product, quantity field is missing:
In Store > Configuration > Catalog > Inventory seems all ok
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You can have MSI enabled, "Multi Stock inventory" then in product edit view there won't be quantity field but a separate tab to handle stocks. The first one under general product fields, and you need to set up sources first to make it work.
Or you can have many stores and default configuration not apply to this one and "Manage Stock" is actually set as No.