I think there was an issue that may this in some early versions. Can you upgrade to the latest version and try again? And if you still see it after the upgrade, please file an issue for it.

I think in your case it's hard to decide because losing TUI could be a big drop in productivity (I actually proposed TUI support for ruby/debug but it's too big of a maintenance burden). But as I said in the end of the post, ruby/debug is going to evolve faster, especially with newer Ruby versions. So even if you don't plan to migrate now, I still recommend you to try out the ruby/debug features I mentioned, and see if they can benefit you in the long term. I believe in the next year or two someone will make a TUI support for ruby/debug too.