Yahoo!メールのIMAPでのメール削除絡みの不具合の件、調べてみた。
多分Yahooのメールサーバーがuid expunge ***(RFC2359)に対応してない。
K-9のアップデートでこのスレ覗いただけ(FairEmail使ってない)のと
英語できないので誰か説明文追加と下のログをsupportフォームなりに報告して。

// error (not deleted)
? UID search all
* SEARCH 9233 89235 89236
? OK UID SEARCH completed
? UID expunge 89236
? OK UID EXPUNGE completed
? UID search all
* SEARCH 9233 89235 89236
? OK UID SEARCH completed

// success
? UID store 89236 +FLAGS (\Deleted)
* 3 FETCH (FLAGS (\Deleted \Seen $NotJunk) UID 89236)
? OK UID STORE completed
? expunge
* 3 EXPUNGE
? OK EXPUNGE completed
? UID search all
* SEARCH 9233 89235
? OK UID SEARCH completed