I have localized the issue to includes/form-tag.php in the get_id_option() function. The static $used variable is incorrectly populated with the current field ID before it is used for the first time. I haven’t really dug deeper into the why’s as I don’t have time right now, but for anyone else waiting for a patch, you can simply remove the static keyword (line 118) to make $used a local variable and bypass this behaviour altogether.
I am experiencing this as well with the latest update, no field ids are propagating into the HTML for any forms on a number of client websites.
