7.0 on TB 91.2.0 (32 bit) on Windows 8.1 (64 bit):
With View Message Body As = Plain Text, and Allow HTML Temp preferences set to Plain Text (with "Always show attachments inline" deselected) display is as intended (plain text) until Show HTML is used once. Thereafter any message with HTML content shows as HTML. Preferences have been reset per subject of this thread, although status popup menu still shows Plain Text as selected option.