Muxing MKVs in MKVToolNix 31.0.0 loses video quality

Hey guys, I did a little reading on different forums and saw some posts from several years ago, but none with a definitive answer (maybe there isn't one?). I got MKVToolNix 31.0.0 thinking that I would be able to strip away subtitles and unwanted audio away from MKV files, without affecting video quality. I started noticing that when I organize my newly muxed MKVs (subtitles and audio stripped) in Plex, the video bitrate seems to go down. I read that MKVToolNix doesn't do any encoding/re-encoding so, in theory, the video quality should not be affected. But my tests, and a few other reports I've read on forums, leaves me with questions on this.

I conducted about 5 experiments on the same file to test this out. I was hoping that if I gave you guys enough details, you guys might have a couple answers/theories as to why the video bitrate goes down if I'm simply stripping unwanted audio and subtitles. So here we go :cool:

Computer

MacBook Pro TB 2018
macOS Mojave
MKV ToolNix 31.0.0 (Recently updated from 30.0.0)


Source MKV File Tested

[Attachment 48250 - Click to enlarge]

[Attachment 48257 - Click to enlarge]

The Equalizer 2
File Size = 16.17GB
Video Tracks = 1
Audio Tracks = 2

• TrueHD Atmos
• AC3

Subtitle Tracks = 17

• HDMV PGS = 16 (Various Languages)
• SRT = 1

So I tested muxing this 5 times. I muxed it twice without changing any settings to make absolutely sure video quality or size would not decrease... and it didn't, which is good. But the others, I had unexpected results, and here they are...



First Mux Test
Didn't uncheck any boxes. Left File as is.
Result: No change in file size [16.17 GBs] or Video size [19135 kbps]

[Attachment 48252 - Click to enlarge]


Second Mix Test (Same)
Again, didn't uncheck any boxes. Left File as is.
Result: No change in file size [16.17 GBs] or Video size [19135 kbps]

[Attachment 48253 - Click to enlarge]

Third Mux Test (Same)
Last time for sanity check, didn't uncheck any boxes. Left File as is.
Result: No change in file size [16.17 GBs] or Video size [19135 kbps]

[Attachment 48251 - Click to enlarge]

Great, so the Muxes were quick, and no file size or video quality was reduced. Now I changed things...

Fourth Mux Test
Unchecked all foreign language subtitles. Unchecked AC3 Audio. Left only 2 HDMV PGS subtitles, and the TrueHD Audio track.
Result: Smaller file size [15.5 GBs] (expected), and smaller video bitrate [18351 kbps] (unexpected).

[Attachment 48255 - Click to enlarge]

So I thought maybe the PGS Subtitles had something to do with it...

Fifth Mux Test
Unchecked only AC3 Audio. Left all HDMV PGS and SRT subtitles, and the TrueHD Audio track.
Result: Smaller file size [15.63 GBs] (expected), and smaller video bitrate [18494 kbps] (unexpected).

[Attachment 48256 - Click to enlarge]

So to sum it up, when I strip anything away, it seems the video bitrate is going down. My question for your brilliant minds. Why is the Video Quality decreasing if I'm not re-encoding anything? Any help is appreciated guys. Thank you in advance! :)