More then likely this is due to the converter limit valve being blocked or stuck closed. This is not uncommon as you have clutch wear and the clutch material starts floating around the transmission. When you go to wide open throttle the ECM commands the transmission to hold in gear but as the pressure builds, it automatically kicks to another gear. When this starts to wear the bypass for the limit valve get plugged and the transmission will stick in gear or even overheat due to such high RPM.
When you shift manually you are taking this function over and are changing the position of the manual valve in the valve body which will change the flow of pressurized fluid.
Let's start checking the fluid condition and level. If it is questionable, change it.
If the fluid is ok, then unfortunately the only solution I would think is to make sure there are no codes and then I would go into the transmission by dropping the pan and seeing what is inside the pan. You may find a bit of metal which would indicate you need to rebuild the unit.
However, before you go to this point, I would just make the transmission last as long as possible. Limit the amount of time you go to WOT because in all honesty, you don't have to do this, that is just your driving style. I don't remember the last time I went to WOT and I have a 2020 Ram Rebel, with a Hemi :)
If you do go to WOT, instead of getting completely off the throttle, trying just letting up a little and I suspect it will shift. If this is the case then this is your issue and hopefully new fluid will solve it for you. After you change the fluid you may need to drive it 100 miles before you see an improvement.
I attached below the process from the manual on how to inspect the fluid if you want to review that. Let us know what questions you have. Thanks
Images (Click to enlarge)
Saturday, February 6th, 2021 AT 12:10 PM