I’ve just tested it on my end but failed to reproduce the issue. The one that should get the
<b></b> tag is the post title, not the thumbnail.
Maybe there’s a conflict on your site. Could you please try to reproduce the issue on one of the WordPress’ stock themes (like Twenty Sixteen) with other plugins deactivated? If it disappears, then you can activate the theme and plugins one by one to see which one causes the issue.
Also, could you please tell me the ACF version you are using?
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