Steps to reproduce the bug

1. Navigate to https://cryptii.com/pipes/alphabetical-substitution
2. Use the substitution cipher:
   abcdefghijklmnopqrstuvwxyz
   ﾁｺｿｼｲﾊｷｸﾆﾏﾉﾘﾓﾐﾗｾﾀｽﾄｶﾅﾋﾃｻﾝﾂ

Select Decode, and enter some katakana in the left box. It'll be copied over to the right box with no transforms.
Encoding alphabetical text does transform it into katakana.
You can also reverse the cipher to
ﾁｺｿｼｲﾊｷｸﾆﾏﾉﾘﾓﾐﾗｾﾀｽﾄｶﾅﾋﾃｻﾝﾂ
abcdefghijklmnopqrstuvwxyz

and Encode katakana and the same no-change copy will happen.

Version, environment and state

I can't seem to select the text inside the alert box, sorry.