In the last specification, palette_coding() is specified as below:

palette_coding( x0, y0, nCbS ) {
  ...
  PaletteRun = nCbS * nCbS − PaletteScanPos − 1
  ...
  if( MaxPaletteIndex > 0 ) {
    if ( CopyAboveIndicesFlag[ xC ][ yC ] == 0 )
       remainingNumIndices − = 1
    PaletteMaxRun = nCbS * nCbS − PaletteScanPos − 1 − remainingNumIndices − copy_above_indices_for_final_run_flag
    if( remainingNumIndices > 0 || CopyAboveIndicesFlag[ xC ][ yC ] != copy_above_indices_for_final_run_flag )
      if( PaletteMaxRun > 0 ) {
        palette_run_prefix
        if( ( palette_run_prefix > 1 ) && ( PaletteMaxRun != ( 1 << ( palette_run_prefix − 1 ) ) ) )
          palette_run_suffix
      }
  }
  ...
}

When PaletteMaxRun is equal to 0, no palette_run_prefix is parsed. There is no inference rule for this syntax element.
In this case, PaletteRun is not derived from palette_run_prefix and palette_run_suffix.

It is set to default value: nCbS * nCbS − PaletteScanPos − 1; as specified above in the parsing.
However, it seems that PaletteRun should be set to 0 according to the semantic of PaletteMaxRun
(7.4.9.13 Palette semantics: "The variable PaletteMaxRun represents the maximum possible value for PaletteRun").
In the HM, PaletteRun is set to 0 in this case.

What is the right value for PaletteRun when PaletteMaxRun is equal to 0:

• nCbS * nCbS − PaletteScanPos − 1

or

• 0 ?

In the same idea, the variable PaletteMaxRun is derived before checking the following condition:

 remainingNumIndices > 0 || CopyAboveIndicesFlag[ xC ][ yC ] != copy_above_indices_for_final_run_flag

However, when this condition is not verified, the variable PaletteMaxRun is not used and could violate the condition:
(7.4.9.13) "It is a requirement of bitstream conformance that the value of PaletteMaxRun shall be greater than or equal to 0."

Indeed, it is possible to have PaletteMaxRun equal to -1 when these 3 conditions are filled:

• PaletteScanPos = (nCbs * nCbs) - 1
• remainingNumIndices = 0
• copy_above_indices_for_final_run_flag = 1

This case appears especially in HM generated streams.

Is this case a legitimate one ?
If, as I believe, it is, this constraint needs to be verified after the condition.
The easiest way to do this is to move PaletteMaxRun derivation after the condition, as following:

 palette_coding( x0, y0, nCbS ) {
   ...
   PaletteRun = nCbS * nCbS − PaletteScanPos − 1
   ...
   if( MaxPaletteIndex > 0 ) {
     if ( CopyAboveIndicesFlag[ xC ][ yC ] == 0 )
       remainingNumIndices − = 1
       if( remainingNumIndices > 0 || CopyAboveIndicesFlag[ xC ][ yC ] != copy_above_indices_for_final_run_flag ) {     
         PaletteMaxRun = nCbS * nCbS − PaletteScanPos − 1 − remainingNumIndices − copy_above_indices_for_final_run_flag
         if( PaletteMaxRun > 0 ) {
           palette_run_prefix
           if( ( palette_run_prefix > 1 ) && ( PaletteMaxRun != ( 1 << ( palette_run_prefix − 1 ) ) ) )
             palette_run_suffix
         }
      }
   }
   ...
 }