I will repost the walkthrough I made here for ease of viewing.
First issue shows that the brights are not bright, and the system calibration text is incorrect from all default settings on MUSHclient.
Click on 'Game', navigate through 'Configure' and click on ANSI Colors (Alt+6)
Make sure to tick the highlighted box here.
Click on Custom Colour (Alt+8), and look to the Custom16 box. This is what we will be changing.
Click the blue square and scroll through the color picker to the top. Then, click on 'white'. After that, click OK, and OK a second time at the bottom of the original window.
Finally, navigate to the output tab (Alt+5) and make sure these boxes are ticked.
When you run the command again, calibration is successful!