Hi @jpcapone,
Set DisableDocumentLibraryDefaultLabeling to true will also disable the DefaultLabeling, you could run following cmdlet to enable the feature
Set-SpoTenant -DisableDocumentLibraryDefaultLabeling $false
And if a library is enabled for IRM, you won't be able to select a default sensitivity label, you could check the following requirements
https://learn.microsoft.com/en-us/purview/sensitivity-labels-sharepoint-default-label#requirements
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