Slight quality loss with export

It seems like there is a slight drop in sharpness in my exported video compared to the original footage. Kind of like generational loss when copying VHS tapes. I'm using Vegas Pro 23 and with regards to my export settings, I'm using the same resolution and frame rate as the original footage (1920 x 1080, 25fps.) The bitrate is close to the original and I'm using a constant bitrate. Looks like the codec is h264. The name of the export option that I used is "Internet HD 1080p 25fps."

The original footage was recorded with a Panasonic G6 M4/3 mirrorless camera and has pleasing sharpness. It appears to be an MTS file. 

I did use masking on the timeline but I doubt that would make any difference to the sharpness of the exported file. There was no digital zooming in or anything like that. Also, I did not adjust the contrast at all and no colour grading either. 

Any idea why I would be getting a slight drop in quality?

HI Silver Floor.  It might be helpful for Derek and other members if you are able to provide Mediainfo data for both the source file and the rendered file.  Derek has a tutorial  if your not familiar with the app.
Cheers, Robbie