You're onto something. It seems that if the beat numbers go high enough, it will be ok up till 16th beat and 17th is missing the grid line. This seems to repeat in 16 beat cycles, so the missing ones go 17, 33, 49 etc.
The ruler numbers are also missing in those places, if ruler is set to Measures.Beats or Measures.Beats (minimal). MIDI editor seems to show these just fine, no problem there.
Same happens with Reaper v5.99, so this is an older thing. I made a
bug report about this.