Dear @abolam and everyone else experiencing the issue,
It seems like Microsoft still haven't resolved the issue of missing forms. The issue appears on mobile devices, as well as in Microsoft Teams desktop application - the forms are not being rendered properly. Microsoft are aware of the issue, but we've seen no updates on this matter - https://github.com/SharePoint/sp-dev-docs/issues/4968
Since the issue is quite critical, and since it now appears even in Standard version of SharePoint, we recommend using PowerShell scripts to fix the problem with existing forms.
The following commands will allow you to install PnP Powershell, connect to any SharePoint site and switch page type for Forms, so they actually render on mobile devices:
Connect-PnPOnline -Url https://domain.sharepoint.com/sites/sitename
Set-PnPClientSidePage -Identity "PlumsailForms/ListName/Item/NewForm" -LayoutType Article
If the issue is not resolved soon, we'll switch all our forms to this page type. This is not ideal, but at least this works, unlike Single Web Part pages.