Model uploads would benefit from being given a default category during upload. Models uploaded directly to Shapeways using the "Upload 3D model" button don't appear in the inventory list until you assign a product category and that can cause you to lose track of what's been uploaded, particularly if you use the inventory list to keep all your uploads sorted and categorized. Tinkercad, as an example, has been doing this forever and assigning the Maker/DIY category using the API when you upload from their system.
Inventory seems to be buggy. Old product has category and shop section... but disappeared from inventory weeks ago. Maybe adding one those triggers something else, just masking the real issue.
There is much bugginess around here! But I wouldn't mind a default category being auto-chosen just to be safe. You have to pick one eventually and if you're lucky it may be the defaulted one.