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Having to retype the search phrase is just a plain old ux bug

No, it starts by searching the emails that it has loaded locally first, and then there is a button if you want to search on the server.

I get this, but most email is through Google and Microsoft in NA, at least, neither of which seem to care to support JMAP at all, on fact they are getting locked down more and more.

Obviously they want you to use the web interface so they can show you ads and gather more data about you (for ads).

JMAP support turns out to be a strong signal for the business model of an e-mail provider. IMAP they have to have, JMAP is for those who actually care about supporting local client software.