Welcome to the forum.My 2019 Renegade latitude wont switch to 8th or 9th gear on its own doing highway speeds either, also if I manually switch it to 9th, it wont hold speed, itll start to slow down, I seem to get way lower mpg than I should be but I never really thought it was something wrong with the car until I started reading the posts on this forum
The road needs to be pretty flat for the Renegade to stay in 9th gear and no headwinds either. When my 2015 Latitude is in 9th gear the rpms are around 1500, that is pretty low. Tailwinds are nice. Mine will easily go into 8th around 60 mph and will run ok down to 55 mph before it downshifts into 7th gear. The rpms are around 1700-1900 in 8th. I personally feel the Renegade would have been better off with a 7 speed transmission.
The Latitude should easily go into 8th without manual shifting. I wonder if there is a software update for 2019's? If you get it checked out keep us posted.