I'm not sure it only happened to me, but after the migration(kagi.com/assistant -> assistant.kagi.com), any new bang of custom assistant or update of old bang both seemed not working.
At first, I made a new bang for a new custom assistant.

and I typed !tata in my address bar as usual, and kagi redirected to https://kagi.com/search?q=%21tata which is unexpected redirection.
Second, I had another try. Update bang(it's working well) of old custum assistant(created before migration) to "!blah", and redirected to https://kagi.com/search?q=%21blah. Tried old one not "!blah" it's still working.
Conclusion is "The UI I see is not what I get".
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I also tried fill in "!tata" on bang setting. Not just "tata".
I expected to see the page of my custom assistant I set for bang.