If you have the search form override activated and still the default search forms appear, that means those are hard coded into the corresponding theme files – and are not possible to replace automatically.
The only way is to manually find the template files with the forms, and replace them with the search PHP shortcode. For that you might have to ask the theme developer to find it.
Alternatively, if you want, I can look it up and replace it for you if I can. I will need back-end and FTP access for that, and I can't guarantee that I'm able to do it, as it's beyond the plugins scope. So far I was able to help in similar situations, so I think it's definitely worth a try 🙂Best,
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