Found a possible bug
.
At 23.976 fps playback, the display seems to upcount a bit too much further down the timeline. Try in at 20-25 minutes to see the effect, comparing the transport clock to the one onscreen.
We really should ask Justin to give us a few simple functions to read out the current clock in an already formatted string. They had to work quite hard to get the clocks correct in Reaper during the past few years.