MIME header decoding failure or character encoding regression causes mojibake in subject lines

Checklist

• I have used the search function to see if someone else has already submitted the same bug report.
• I will describe the problem with as much detail as possible.

App

Thunderbird for Android

App version

26.0

Where did you get the app from?

Google Play

Android version

16 (One UI 8.5)

Device model

Samsung Galaxy Z Fold7

Steps to reproduce

1. Send yourself an email with an emoji or non-Latin character in the subject line
2. Put an emoji or non-Latin character in the body of the email
3. Receive the email in Thunderbird for Android

Expected behavior

Email should arrive with all emojis and non-Latin characters properly represented in the subject line and the body.

Actual behavior

Subject line shows garbled text or mojibake instead of the intended emoji or non-Latin character. The same characters are represented properly in the body of the email.

The problem may also be affecting the "From" header.

Emails with emojis in the subject line received in previous versions of Thunderbird still show with the emojis properly represented.

Logs

No response

Reproduce the issue in Thunderbird for Android 26.0 using emoji and non-Latin characters in subject lines, then compare the subject and From headers with the correctly rendered body. Trace the MIME header decoding path and review behavior from versions where existing messages rendered correctly. Done means affected headers display the intended characters without regressing body decoding.