Hi there,
Thank you for reaching out on the community, this had been a limitation of the Path Tracer since introduced in Twinmotion 2022, as referenced in our Twinmotion 2022.1 Preview 1 Release Notes here.
However, decals should now be supported when using the Path Tracer in our next Twinmotion update, which is right around the corner. :)
Please stay tuned as we will be posting our update announcement to the Twinmotion Community here, along with the update's Release Notes, which will contain more information on fixes and known issues.
Best Regards,
Morgan