I'm running fish 4.0.8 on macOS 26.0.1.

I'm trying to implement an automated test for Fish completion for some 3rd-party tool. When I run Fish interactively, I can run source path/to/tool/share/fish/vendor_completions.d/tool.fish and then complete -C 'tool ' outputs the completions as expected. When I try to run it in a test with fish -c 'source path/to/tool/share/fish/vendor_completions.d/tool.fish; complete -C 'tool ', I get nothing.
The tool uses @pnpm/tabtab to generate completions that uses this template: https://github.com/pnpm/tabtab/blob/main/lib/templates/completion.fish. Notice how it uses commandline -C to get cursor position.

When I try to test it from command line with fish_trace, I get this output from the beginning of that function:

> complete -C 'tool '
-> _tool_completion
----> commandline -o
---> set cmd tool
----> commandline -C
---> set cursor
----> count tool
---> set words 1

When I try it interactively, I get this:

> complete -C 'tool '
-> _tool_completion
----> commandline -o
---> set cmd tool
----> commandline -C
---> set cursor 0
----> count tool
---> set words 1

And this makes all the difference for the tool's completion machinery. Is it possible to make commandline -C return something when running in such manner? Should I run it differently to test completion?