If you expand the attack (click on it to bring up the sidebar) I think you will find that there is an additional line for necrotic damage.
Right now it is not possible to change the damage die on homebrew items. It has been requested multiple times. Hopefully the functionality will eventually be added. The more people who ask for it, the better chance it will be added to the roadmap and/or moved up in priority.
This is the way I assume you would replace the damage die (based off pistol weapon [non-Exandria]), but this is what shows up on the character sheet:
If you expand the attack (click on it to bring up the sidebar) I think you will find that there is an additional line for necrotic damage.
Right now it is not possible to change the damage die on homebrew items. It has been requested multiple times. Hopefully the functionality will eventually be added. The more people who ask for it, the better chance it will be added to the roadmap and/or moved up in priority.
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It does successfully convert to necrotic, but as you said, the damage die is not replaced.