Markdown: double underscores inside emphasis can truncate the span

Prettier 3.9.6

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--parser markdown

Input:

*a__b__c*

Output:

_a__b_\_c_

Expected output:

*a__b__c*

Why?

The entire identifier should remain emphasized; equivalent delimiter choices are fine. CommonMark 0.31.2 renders the input as <em>a__b__c</em>, but the output as <em>a__b</em>_c_. The emphasized key__x__value sentence also fails; the corresponding key__xy__value control preserves meaning.

Also reproduced on development commit 891accaaab6dab0fbb5eba6748fbb1f5ac4809b5. Related #3834 concerns word-leading underscores inside strong text; this example uses intraword underscores inside emphasis and introduces an escape despite a correct input AST.

GPT-6 Astra assisted; I manually reviewed this report.