DKIM signature is sometimes invalid

I am running currently stable cPanel v76+, with exim 4.91. DNS is hosted on Cloudflare and I can mostly successfully send a properly DKIM signed emails with a score 10/10 on mail-tester.com. But sometimes messages from the exact same email result in invalid DKIM signature according to Gmail and mail-tester.com and I cannot figure out why. Manually comparing valid and invalid DKIM emai headers shows this. In the copy paste of headers I've hidden domain and is a correct one in headers received by Gmail or mail-tester.com. Valid DKIM signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=hidden.tld; s=default; h=List-Unsubscribe:Content-Type:MIME-Version:To:Reply-To:From:Subject:Date:Message-ID:Sender:Cc:Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:In-Reply-To:References:List-Id:List-Help:List-Subscribe:List-Post:List-Owner:List-Archive; bh=033XepfoqR3mPbERwV3ZBIINwWfhsAr6KS/n+cYEm/g=; b=BvdRcfqrdKcX5/c10pr8IeLNvq7U1VL8w5qRaHfwsyFRS+bvfKI4tLa+tX+0kuMpChVK5I2k1vcfj2BZPu2JEnvPLF8SSShjmYeMvBY8wCQbTfNq5YMl9fFluxTt5bi2G2MhdeSRxsURQ/W6l+Rtbbp/unPOpoOfq+tCbUha4c0KdlpJO6ArhyffcuNiJlzy8iAZNDbo6x6gxG+olS4Gbh9x96HN8tlLJw3bhvg2l17pPXnLwQALN5z7R7fE0RVefjLOGw/11SYJBAxKyZfhbyNZXNE5vbb3dKv8JLU34dJSHsotq+2z5SvIuIJG3nXlGir60EU/7a9HGa40BMrRDQ==; This key is obtained from dns query to validate the email as valid: "v=DKIM1; k=rsa; p=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAzsBIqubAjZEgzpGB5qC9KZhhKpGbevYtQXI+0opmGaoiPYlRPGY4UEe157LJRWrv4RdO31jErFV7jmI6nkdk0dqFQM8gIoMFC8vmNNzp3vZrionAgsGMqS7EyuFxcWCqZPBLCigFb5CanPGgNou3qaqxsGHQnjem6HQTRzKa6Pu4M31uUJyrW5wJ7JTKxfNmzJ5a8r/rFH0IDfziJOzpihkO+97nSGcEIhtCFKsL2+TKDJGACN0c3YY4MOv3yazM/MlmTp/QVaA9xKBbOp5UueDKkTPB0nK/kldR4rx21vltX65yWaM2fbUwx7aPFCpM0pOQ7ah/g6N09mhalzY5UwIDAQAB; " Same message in only a short time difference validates as invalid DKIM signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=hidden.tld; s=default; h=List-Unsubscribe:Content-Type:MIME-Version:To:Reply-To:From:Subject:Date:Message-ID:Sender:Cc:Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:In-Reply-To:References:List-Id:List-Help:List-Subscribe:List-Post:List-Owner:List-Archive; bh=Rds/2G37+Wp1V7pPNyNErWIEGvOJ9zA7A8qhkd4nCuE=; b=E6x5aGoU0tlqnVw9W53yUQYOMDP9RDra9GhdVzyWBwM+1m5VNq12EdIzYaYexoTPryF26mNogfJ5cyYMTDZdoPbqNuhzGoVsHCdNncQhwbQHErf5d2/R0XbYbmWb+i9V6KI8qyo0Ps89pxuknJB1F9Ffpn63YhXo1errdZcgjvkC/umCUq57KSQTWD7CmCDwJ85HFBv9wXYpY1g+7H5Kk+H9zaJLxt7ofAfRRei7EpDEtwWyG8+YaxxVFWgDhjJ/o8IKwpuHybhHxxXhfNCUtz7i8Am2At/awVep4aN9g+nAgxNbIOFeCOQA5j+E3W3hS38/Ft8NwdU7JxIlYKTMXA==; This key is obtained from dns query to validate the email as invalid: "v=DKIM1; k=rsa; p=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAzsBIqubAjZEgzpGB5qC9KZhhKpGbevYtQXI+0opmGaoiPYlRPGY4UEe157LJRWrv4RdO31jErFV7jmI6nkdk0dqFQM8gIoMFC8vmNNzp3vZrionAgsGMqS7EyuFxcWCqZPBLCigFb5CanPGgNou3qaqxsGHQnjem6HQTRzKa6Pu4M31uUJyrW5wJ7JTKxfNmzJ5a8r/rFH0IDfziJOzpihkO+97nSGcEIhtCFKsL2+TKDJGACN0c3YY4MOv3yazM/MlmTp/QVaA9xKBbOp5UueDKkTPB0nK/kldR4rx21vltX65yWaM2fbUwx7aPFCpM0pOQ7ah/g6N09mhalzY5UwIDAQAB; " I have ruled out DNS server, as I have complete trust in Cloudflare, could it be exim's fault?