Hi @Mickael Dubois ,
If you are unable to see the app registration you can go to Enterprise Applications > Your app > Properties. There you will see a note that says "If this application resides in your tenant, you can manage additional properties on the application registration."
If you click on that link from the Enterprise Application > Properties page, you should be routed to the Branding and Properties page where you can configure the app registration settings:
If you do not link to this page, the app might not be registered in that tenant or the registration may be removed. I also know of one change to the Azure portal that was causing older registration to not show in some cases, so it could be related to this same issue.
If you send me an email at AzCommunity@microsoft.com ("Attn: Marilee Turscak") and include your subscription ID and a link to this thread, I can get a one-time free support case opened to look into this issue.
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