I don’t really have any intelligense specific to RPB but on the hardware side… the video hardware in the SoC of both Hikey and Hikey960 has a fairly inflexible pixel clock PLL which restricts the range of pixel clocks in can generate.
In the AOSP code base there are some hacks to recognise some CEA-861 video modes that the hardware can’t hit exactly and mutate the pixel clock to one that does work. 1080p60 is one these modes… with the pixel clock modified the mode is actually 1080p58 and does cause problems with some monitors (typically strict TV-like devices that blank the image if it is “unhappy”).
It looks like your display is monitor like and these tend to be more compatible. However if it possible that the 1080p60 mode ends up out of spec if it does not use CEA-861 timings and isn’t picked up by the hacks to recognise specific pixel clocks (because the PLL code tends to round up).
https://bugs.96boards.org/show_bug.cgi?id=598 contains some useful attachments describing some of the impact.
PS 50Hz cannot workaround these issues… CEA-861 modes are designs so that 1080p50 and 1080p60 have different active areas and hence both use the same pixel clock.