I just discovered one more thing immediately after I posted. It’s not that the the fields aren’t loading — they are — they’re there in the DOM. But the ACF field container is being assigned a hide-if-js class that styles them with display:none;.
e.g. the containing element looks like this: <div id="acf-group_[xxx]" class="postbox hide-if-js acf-postbox seamless">
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