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#21
Well basically since your GPU is mobile then it has Optimus , but the problem is they disable it in Bios (Source : regarding PH517-51-72NU and g-sync and optimus — Acer Community) , with no option to turn G-Sync off then Optimus will never activate .
Then your solution may lay in removing the laptop driver and install the desktop driver despite having a mobile GPU since the main difference in both is Optimus support , this could stop handing out refresh rate management to external GPU that isn't there and keep everything handled by Nvidia control panel .
Fill this out: Display Driver Feedback Form
An nvidia representative may be in touch if they need more information or have a fixed driver for you to try
I have only one final suggestion which seems out of the box , there is a tool named Custom Resolution Utility or CRU , it allows for creating new resolutions and deleting old ones , you may try deleting the resolution / refresh rate that you get dropped to , maybe at its absence it becomes impossible to drop to it .
I am having the same issue since late 2020 guys any updates on this matter?