AMAZON SES - use of dedicated IPs causing undelivered mails

We started using 2 dedicated IP addresses last week (becaouse we were getting classified as spam by a few of our clients servers), with automatic warm up enabled. Ever since that moment, a great amount of mails to Microsoft 365 accounts stopped being delivered. A couple of examples of bounces we are getting (I have bolded what I consider the important bits):

{"notificationType":"Bounce","bounce":{"feedbackId":"01000183ec4e858b-23653b95-dc32-4bc5-8c79-c5576c654ab1-000000","bounceType":"Transient","bounceSubType":"General","bouncedRecipients":[{"emailAddress":"ALVARO.MOLLINEDO@TIENDAAMIGA.COM.BO","action":"failed","status":"4.4.7","diagnosticCode":"smtp; 550 4.4.7 Message expired: unable to deliver in 840 minutes.<451 4.0.0 Unknown>"}],"timestamp":"2022-10-18T18:15:12.000Z","remoteMtaIp":"104.47.51.110","reportingMTA":"dns; a121-81.smtp-out.amazonses.com"},"mail":{"timestamp":"2022-10-18T04:15:07.803Z","source":"avisos.tiendaamiga@salar10.net","sourceArn":"arn:aws:ses:us-east-1:119356527540:identity/salar10.net","sourceIp":"18.214.19.61","callerIdentity":"ses-smtp-user.20210113-144245","sendingAccountId":"119356527540","messageId":"01000183e94d681b-347c8244-c7b1-4435-a54a-e78082cb671a-000000","destination":["karen.cespedes@tiendaamiga.com.bo","favio.quisbert@tiendaamiga.com.bo","merly.ramirez@tiendaamiga.com.bo","javier.pelaez@iqus.com.bo","nicole.calderon@tiendaamiga.com.bo","ALVARO.MOLLINEDO@TIENDAAMIGA.COM.BO"]}}

{"notificationType":"Bounce","bounce":{"feedbackId":"01000183ec57b5a3-5fa6bf1e-4969-47e1-9a2c-9b1da6abdfa4-000000","bounceType":"Transient","bounceSubType":"General","bouncedRecipients":[{"emailAddress":"ACACERES@CLINICADELASAMERICAS.COM.BO","action":"failed","status":"4.4.7","diagnosticCode":"smtp; 554 4.4.7 Message expired: unable to deliver in 840 minutes.<451 4.7.500 Server busy. Please try again later from [54.240.121.82]. (S77719) [BN1NAM02FT058.eop-nam02.prod.protection.outlook.com]>"}],"timestamp":"2022-10-18T18:25:14.000Z","remoteMtaIp":"104.47.51.110","reportingMTA":"dns; a121-82.smtp-out.amazonses.com"},"mail":{"timestamp":"2022-10-18T04:03:25.674Z","source":"avisos.clinicaamericas@salar10.net","sourceArn":"arn:aws:ses:us-east-1:119356527540:identity/salar10.net","sourceIp":"18.214.19.61","callerIdentity":"ses-smtp-user.20210113-144245","sendingAccountId":"119356527540","messageId":"01000183e942b16a-c9696617-8145-411f-b1bb-f898d07685c1-000000","destination":["ACACERES@CLINICADELASAMERICAS.COM.BO"]}}

Some useful information:

• Before using the dedicated IPs, we weren't getting any of these bounces.
• Message count and rate has not changed.
• In addition to adding the dedicated IPs, we also configured a custom MAIL FROM for the subdomain, following AWS instructions to the letter.
• Messages are NOT being directed to spam folders in MS 365 accounts, they do not reach destination.
• We have tested our mails with a couple of online mail testers, and we get very good scores (>=90%).
• We have w h i t e l i s t - e d both the domain and the IPs in MS 365 exchange admin center, to no avail.
• Reverse DNS works fine.

¿What can we do? ¿If we relinquish the dedicated IP addresses currently being warmed up, should everyting go back to normal? ¿Contact Microsoft support and hope they can provide a reason for those bounces, so we know what to do in order to solve the issue?

Thank you very much in advance for any help.