It is because the overdrive is applied with the underdrive in third and that opposing force will lock it up and cause the 1:1 ratio. However, when you remove that opposing force the overdrive is slipping.
They should be getting some codes. If you can get those, that will help confirm this before they pull the unit back out. At least at that point we will know exactly what to go after if they have to send it back.
Take a look at the attachments below and look at the third gear vs fourth gear and you will see where in 3rd they are applying UD and OD that are basically opposed from each other.
Let us know what the codes are and we will know for sure what is happening.
Images (Click to enlarge)
Sunday, January 24th, 2021 AT 6:04 PM