This seems to be a regression recently introduced as not reproducible on the WordPress.com desktop app.
I didn't expect to see a warning that 'baseball' is taken as baseball isn't a TLD
Warning shown that baseball is taken
Chrome macOS

Actually, .baseball _is_ a TLD (part of the very wide range of new TLDs...). It's not offered on wpcom yet, so like you said, this is very much a regression, so thank you very much for bringing this to our attention 馃檱
I've reproduced this on Domain Search in Domain Management too - search for an unsupported TLD and then search for any keyword, you'll get some weird notice:

@delputnam, is this something you could have a look at?
Yep. I'll look into this today. I think I know what the issue is.
Thanks for the quick fix 馃檹