Our 2020 does this and is my biggest complaint with the truck. It's also the reason we had to remove the TRD Pro exhaust from the truck as the constant hunting induced massive and resonating cabin drone that was terrible on highway trips.

Even in early 2000s cars and trucks, this shifting behavior is not acceptable and is a well-known Toyota transmission logic tuning issue. When driving on the highway and without cruise control, the torque converter will unlock in 5th, thus allowing the truck to handle far steeper grades. With cruise control is engaged, the truck will downshift into 4th on almost any grade. Better yet, the thing will stay in 4th well past the grade and often times requires me to lightly stab the throttle to induce the upshift. What's really funny is when you hit just the right minor amount of grade and trans will go 4th-5th-4th-5th-4th-5th until you get out of the grade. My wife and kids use to think I was doing it myself and screwing with them.

How Toyota has not been able to remedy this absolutely awful shifting strategy and behavior is unacceptable.

If we're driving in hilly areas or areas with long grades, I often times just leave the truck 4th. It's a far more relaxed drive and at speeds below ~75mph running in 4th in areas such as these doesn't result in a measurable mpg impact.