@Cem Karatas Thanks for your patience on this. I have checked with team and sharing the below.
Upon investigating further, our team has identified a bug, a fix has been created and it will be get deployed in next few weeks.
Also, note that the above-mentioned timeline is not exact ETA, and this might vary based on different factors.
Meanwhile as workaround, instead of sending the notifications from portal try using the debug send Rest API with content type as text/xml.
Hope this helps, let me know if you have any further questions on this.
Please accept as Yes if the answer provided is useful , so that you can help others in the community looking for remediation for similar issues.