This can be worked around by putting the path to ignore in the exclude config parameter, but will mean that path is also not considered when building the site.

I encountered this bug on Windows 10 with Jekyll 4.3.3. I used this configuration setting in _config.yml:

livereload_ignore: [ '/assets/images/qrcodes/*' ]

When my generator writes files into /assets/images/qrcodes/, a reload is triggered. I need these files to be part of the site, so I cannot exclude them. Thus, the workaround suggested by @wlabarron is not an option.

The problem is the call on line 161 of lib/jekyll/commands/serve.rb to Jekyll.sanitized_path requires 2 arguments, but only one was supplied. Either this feature was never tested, or the method signature for Jekyll.sanitized_path had a breaking change that was not properly tested:

File.fnmatch(filter, Jekyll.sanitized_path(p.relative_path))

The comment and method signature are:

    # Public: Ensures the questionable path is prefixed with the base directory
    #         and prepends the questionable path with the base directory if false.
    #
    # base_directory - the directory with which to prefix the questionable path
    # questionable_path - the path we're unsure about, and want prefixed
    #
    # Returns the sanitized path.
    def sanitized_path(base_directory, questionable_path)

One easy way to fix this bug would be simply to avoid the call to Jekyll.sanitized_path.
This seems reasonable because I do not believe the call adds value.
Thus, we are left with:

File.fnmatch(filter, p.relative_path)

I tested this and it worked.

Incidentally, the Jekyll docs should mention that the contents of the livereload_ignore array are governed by the rules for the first argument of File.fnmatch.