No there is no bug. You may find that emails with the header "Content-Type: text/plain; charset="utf-8"" use the font data set in 'Fonts for: "Other writing systems", even if the message content is just plain English. As a heads up, next time you have an issue of any sort, please use the Thunderbird Support Forum first to see if a fix can be sorted or whether the problem is something that can be replicated.
Bug 1849284 Comment 8 Edit History
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No there is no bug. You may find that emails with the header "Content-Type: text/plain; charset="utf-8"" use the font data set in 'Fonts for: "Other writing systems", even if the message content is just plain English. There is a difference between western and unicode. As a heads up, next time you have an issue of any sort, please use the Thunderbird Support Forum first to see if a fix can be sorted or whether the problem is something that can be replicated.