If the beat numbers go high enough, grid lines 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). Same applies for the inline editor, but MIDI editor seems to show these just fine.
Happens already with Reaper v5.99, so this is an older thing.
Steps to reproduce:
1. Set main arrange ruler to Measures.Beats or Measures.Beats (minimal)
2. Insert a tempo/time signature marker.
3. Set time signature to 20:4.
4. Check the 17th beat.
It will look something like this:
https://forum.cockos.com/attachment....3&d=1586511931
Discussion:
https://forum.cockos.com/showthread.php?t=234218