Two things to try (and I have no idea whether they will work).
Firstly, the box is not , in fact, a textfield (although that’s how you have it set up)…wht not just change it to an
<input type="email" name="email" required>?
Secondly, since it appears that the issue ‘goes away’ when the field receives focus why not add the
autofocus attribute (whether you change it to an email field or not).
<input type="email" name="email" required autofocus="autofocus">