Samsung S-series has the capability to switch between resolutions. With the coming of the 120Hz display, it may not be possible to see the refresh rate on the highest resolution. This is all going to start from the Galaxy S20 itself.
Although it is possible to get the full 120Hz refresh rates on the FHD+ settings, this might have other significance. Remember how Google underclock its display from 90Hz to 60Hz on Battery Saver Mode? It might be the same in the case of Galaxy S20. First, the highest resolution: WQHD+ enough to suck up some power from the battery, then comes this 120hz display, which will chop off more power.
And this might be the case for the other manufacturers like OnePlus who are also going to use the 120Hz displays from Samsung on their smartphones. There are more smartphones to come this year, where most of them will feature a higher refresh rate display.