@Ickes, Jeffrey Thank you for reaching out to us, As I understand you are creating a payloads within Microsoft Defender portal.
I have reviewed this https://learn.microsoft.com/en-us/microsoft-365/security/office-365-security/attack-simulation-training-payloads?view=o365-worldwide where its mentioned Global payloads are built-in and non-modifiable payloads.
Custom payloads needs to be created under Tenant payloads tab, would request you to review this article and see if it helps to resolve your issue or not, feel free to post back.
I would recommend posting the same query on Defender for Office as well to get more insights on this issue. https://techcommunity.microsoft.com/t5/microsoft-defender-for-office/bd-p/MicrosoftDefenderforOffice365