see How to reproduce.

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Work in pine with charset set to koi8-r.

The attached mbox contains two messages. When viewing the first one, it is autorecoded from \"Windows-1251\" to \"koi8-r\".
After forwarding it, the edited message is OK. But in the resulting message we get charset set to \"Windows-1251\" again, although it has been recoded to \"koi8-r\". This can be seen in the second message in the mbox.

Replying works fine.

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pine-4.55L-alt1

This doesn\'t apply to forwarding any recoded message. It probably has to do with a special representation of the message under consideration: internally, it is regarded as a multipart message with one part -- the body in \"Windows-1251\". And at a certain stage of sending it, it gets collapsed into a plain one-part message, which gets the charset from the charset value of the part, and it was \"Windows-1251\": the valid charset for recoded message is set only for the main part before this happens, so it gets lost.