Inbound approve emails are not approving the records instead it's updating the target records. You can check the email log, it is sipping the "update Approval Request" inbound action.
Skipping script 'Update Approval Request', email is a reply, and the record it matches is not in the Inbound Email Action's table
Received id XXXXXX Classified as reply to 'null' via watermark 'YYYYY' in message
Release or Environment
Inbound email WaterMark not matching in or missing in the sys_watermark table
we need to check the approval email watermark exists in the sys_watermark table, if it does not exist check, it unable to match the approval email hence instead of approving it will match update inbound action and updating the record.
Check the sent approval email has an "Omit Water Mark" setting enabled. If it is enabled, the instance will not send watermark in the approval email hence it's causing the issue.
Uncheck the "Omit Water Mark" from the notification, refer the document to find the Omit an email notification watermark
If the watermark is available and still approval failing to check the "Random Watermark Support" plugin is disabled, check below :
Use one of the following workarounds:
(Recommended) Activate the Random Watermark Support plugin. This will cause notifications to produce random watermarks, matching the properties in place and making the instance more secure.
If the Random Watermark Support plugin is not required, set the values of the following system properties to false:
This will revert the behavior to use normal non-random watermarks on notifications and for classifying inbound messages.