I don't think the behavior is new. The 10 emails all contain the same 6 unrncoded hex values. Shown here with an old TB88.0a1. The last change was the removal of the default-charset. With that one could still control the display of unencoded characters.
Bug 1779070 Comment 10 Edit History
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I don't think the behavior is new. The 10 emails all contain the same 6 unencoded hex values. Shown here with an old TB88.0a1. The last change was the removal of the default-charset. With that one could still control the display of unencoded characters.