MacBook Pro (15-inch, 2019) with a 4k monitor mis-identifies monitor
It is most likely still the adapter/cable being used. I had the same issue with an adaptor that claims to run 4K@60 and it didn't. I have the same model for work, and a 2017 model hooked to a Samsung U28E590D and I can get 60Hz using HDMI or Display Port. If what you say is true, and it is an apple adaptor, then you may just have an older version of the adapter which does not support 4K as shown here: https://support.apple.com/en-us/HT207806