probably you, ( or the calibre upgrade process) altered some setting that affects conversion, but I can't think which one .
or Amazon have sneaked in a recent change.
I admit to some lack of current expertise here, the last time I needed to use Send Mobi to Kindle via amazon, my calibre was still at 3.x ( or even 2.x) so all I can say for sure is what you already said: that is used to work [ with mobi old, not with mobi new, IIRC]
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