(new) Invalid HMAC only for webook app/uninstall

[Marc_Aurelion]

Hello,

Recently I have experienced an issue with HMAC verification from the webhook app/uninstall. 
When I check HMAC with GET methods or POST method for mandatory webhook (GDPR) everything is Okay.
The webhook uninstall used to work properly buy now I can't verify the HMAC. I checked with several parameters and the verification still fail. 
How can I fix this ?