Form works different on different devices

Hi,
I have a Grav installation that has been working since 2018. Since last September, when I updated the Form plugin to v8.0.5, the site started with some erroneous behaviour.

On some of the forms, when a user would submit the form, nothing would happen. The user was supposed to receive an email but the email was never sent.

I can find no pattern to the problem. Myself, if I try the same form from my computer (no matter which browser I use) everything works as supposed to. When I try it from my phone (Safari), I get the bad behaviour - I don’t receive the email.

Currently, I’m running the following versions:

  • Grav 1.7.49.5
  • Form plugin v8.0.5

Any ideas on what might be the problem here?

I debugged extensively and I was able to check that even when it malfunctions, all POST payload is correct. It simply doesn’t send the email as it was supposed to.