Have now replicated this using a testing website.
What do you mean by “a modal login form from users with iPhones”?
I’m seeing no problems logging in with an iPhone on WordPress 5.4.1, Twenty Twenty 1.2, and iOS 13.4.1 myself.
I have a plug in that provides a modal login (AJAX Login and Registration modal popup – PRO). I tested with Lambdatest screen capture and got this result.
ps Twenty Twenty 1.2 and WordPress 5.4.1. Test (as I have a free account, it seems to be the only one available) on Lambdatest was iPhone 8 with Safari 13.3
Since that is a commercial plugin, we ask that you please go to their official support channel, so you can get support from the people who know it best: https://maxim-kaminsky.com/shop/contact-me/
Forum volunteers are not given access to commercial products, so they would not know why it is not working properly. Other community members who may have faced your issue might be able to help you but your best bet is your product’s developer. Keep in mind we encourage you to use the official support venues, as it allows the developers to be aware of issues with their code and gives back to the community in a more robust way.
I appreciate that if it was a single plug in, but I am also getting reports of a similar error which I can, again, replicate using another Mailing plug-in, again using iPhone 8 with Safari 13.3.

Sorry, slip of the keyboard there, that is the standard site search, not a plug in.
Bob
Please attempt to disable all plugins. If the problem goes away, enable them one by one to identify the source of the problem.
OK – it seems it was the YOP Poll Plugin. Bug reliably reacted to activate/deactivate, so I am chasing them. Still getting used to the WP way of doing things!