There is some delay for the client secret and self-signed certificate to take effect, it is normal.
The specific delay time depends on the specific tenant, let's say big tenant and small tenant here. The big tenant has many directory objects, e.g. users, applications, etc, and the small tenant just has several objects.
Pey my test, the delay time of the big tenant is much longer than the time of the small tenant, the bigger the tenant, the longer the time, from several minutes to several hours.
So in your case, I suppose your tenant should belong to the big tenant, you can also test it in a small tenant by yourself, you will find the client secret and self-signed certificate will take effect soon.
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