Interesting read:

https://support.microsoft.com/en-us/topic/screen-refresh-rate-in-windows-does-not-apply-the-user-selected-settings-on-monitors-tvs-that-report-specific-tv-compatible-timings-0a7a6a38-6c6a-2aec-debc-5183a76b9e1d

I ran into a weird Hz problem once and the only way for me to solve it back then was with this tool:

https://www.monitortests.com/forum/Thread-Custom-Resolution-Utility-CRU