Update / Create Matrix failing with item create Workflow

Has anyone else ever come across this?

I'm having a strange issue with inventory parts where matrix items can no longer be created and existing matrix items can no longer be updated via the 'Update Matrix' button on the item record.

After clicking on the 'Update Matrix' button, the typical 'Mass Update Performed' page loads, except instead it reads: 'Your mass update is complete, and has been successfully performed on 0 records.' Child items can still be edited individually.

It seems like this started after I made a workflow that triggers on the creation of an item record. (item, inventory part, event based, before record load, on create, user interface, condition: role=warehouse manager 2) This workflow sets the value of several fields and selects a custom form.