I tried to convert to HDR a video with constant frame rate of 23.976 and in output I got I video with the following rate:

ID                          : 1
Format                      : HEVC
Format/Info                 : High Efficiency Video Coding
Format profile              : Main 10@L5.1@High
HDR format                  : SMPTE ST 2086, HDR10 compatible
Codec ID                    : hev1
Codec ID/Info               : High Efficiency Video Coding
Duration                    : 10 min 0 s
Bit rate mode               : Variable
Bit rate                    : 4 266 kb/s
Width                       : 1 920 pixels
Height                      : 800 pixels
Display aspect ratio        : 2.40:1
Frame rate mode             : Variable
Frame rate                  : 5.997 FPS
Minimum frame rate          : 3.996 FPS
Maximum frame rate          : 23.976 FPS
Original frame rate         : 23.976 (24000/1001) FPS

which is almost unwatchable.

Using the Dolby Video conversion, the situation is better and I got

ID                          : 1
Format                      : HEVC
Format/Info                 : High Efficiency Video Coding
Format profile              : Main 10@L5.1@High
HDR format                  : Dolby Vision, Version 1.0, Profile 5.0, dvhe.05.03, BL+RPU, no metadata compression / SMPTE ST 2086, Version HDR10, HDR10 compatible
Codec ID                    : hev1
Codec ID/Info               : High Efficiency Video Coding
Duration                    : 9 min 59 s
Bit rate                    : 5 919 kb/s
Width                       : 1 920 pixels
Height                      : 800 pixels
Display aspect ratio        : 2.40:1
Frame rate mode             : Variable
Frame rate                  : 23.976 (23976/1000) FPS
Minimum frame rate          : 23.976 FPS
Maximum frame rate          : 24.000 FPS
Original frame rate         : 23.976 (24000/1001) FPS

Please change your encoding so that the conversion will use the constant frame rate encoding. If you want to keep the Variable rate encoding (something that was used in last century) add at least an option to select constant frame encoding and set this a default for the High Video Quality Output.

Thanks