Seems like Gmail silently messes with messages while they’re being delivered sometimes. But it leaves the original in the Sent folder...
I sent an HTML-only message to myself as a test and it de-base64’d and synthesised a text/plain part and rewrote the message :/
Can only tell this by receiving the message elsewhere and looking at the raw content.
I feel like the basic thing you should do as a mail provider is deliver the message unmodified.
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