I just switched from an Rpi4 running LibreElec to an ODROID-N2 with CoreELEC With the pi4, I never thought about refresh rates; everything would just work, with automatic switching.
With the ODROID, only 50, 59.94 and 60hz works. If I don/t explicitly whitelist those resolutions, and only those, I lose the signal completely whenever I play content with lower framerates. My TV is a Sony A1E, and supports the refresh rates. I am using a different HDMI-cable, but I can’t imagine that being the issue when lowering the refresh rate.