NUnit/JUnit summary implies no colour in output messages
Repro solution in case you need it: ExpectoTest.zip
When running with --nunit-summary, the output contains an ESC character. This causes the Azure Pipelines "Publish Test Results" task to fail:
##[warning]Failed to read D:\a\1\a\TestResults.xml. Error : '', hexadecimal value 0x1B, is an invalid character. Line 7, position 1..
Please see the attached test results file.
Here is how the file looks in Notepad++. You can see the invalid character on lines 7 and 8.

The same thing happens with --junit-summary:
##[warning]Failed to read D:\a\1\s\TestResults.xml. Error : '', hexadecimal value 0x1B, is an invalid character. Line 6, position 293..

Could you try --colours 0?
Yep, that solves it. I guess Expecto should always exclude colors from the test results file?
Yes, exactly. PR? :)
Terribly sorry, I'm completely swamped and have no capacity to ramp up on a new codebase. (I have even just had to withdraw from some OSS projects I maintain.)
By the way, colors in the string is also an issue in the VS debugger (when debugging tests with F5):
