Currently hcaptcha does not show properly in modal forms ( https://www.drupal.org/project/modal_forms β ).
See screenshot: https://i.imgur.com/Lif8yNa.png
The captcha box is simply not displayed, though the captcha description can be seen, and submitting the form results in "captcha not correct."
I tested the default math captcha, and it shows up correctly in modal forms.
When the sam form is opened via "open in new tab" (none modal), it displays fine (screenshot: https://i.imgur.com/0AbkRWx.png).
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