@Chris Nott Thank you for reaching out to us, can you help us with the fiddler trace at the time of issue, would like to review the logout request that is being sent to Azure AD.
Reference: https://learn.microsoft.com/en-us/azure/active-directory/develop/single-sign-out-saml-protocol
Fiddler trace can be shared over email - Please send us an email on azcommunity [at] microsoft [dot] com referencing this issue with a subject line "ATTN:Givary"
Let me know if you have any further questions, feel free to post back.