DT 12 Transmission.

There’s your answer the clutch was slipping while your doing all that maneuvering.
Just like you would be in a manual slipping it to move slow.

 

I bet Detroit sent the truck owner s nasty email about engine overspeed if it is set
up. 2300 is engine redline so just be prepared Detroit gets really pissy about overspeed and warranty. Just saying

 

Hello!
I have dd12 trans on the freightliner cascadia 126-2019. After changed the clach transmission started working with problem between gears from 9th to 10th and from 11th to 12th gears doing neutral couple second and then goes to next gear . All Low gear is working good downshift working good as well no any problem . Looking maybe here is someone who have such a experience to share information haw to fix .
Thank you and hope find solution with your help . Here is my personal phone number 470-343-9157 Michael
call or text or
Email : michaelkob2279@gmail.com