I am an iPhone and iPad user myself and often reply to tickets from these devices. I cannot reproduce this issue when replying to an existing OR filing a new ticket.
Also note that the text entry field does not come from ATS. It's the HTML editor configured on your user account or, if none is configured there, the one selected in your site's Global Configuration.
My experience as an iPhone user is limited to two editors: Joomla's built-in TinyMCE and third party JCE. They both work great on iPhone / iPad and I just tested them right now. I use the former on an on-line test site, the latter on this site right here. I have separate, non-privileged accounts for testing i.e. I am testing as a simple Registered user, not as a Super User.
I'd recommend checking which HTML editor you are using, whether it supports mobile authoring and your configuration of the HTML editor. Also check whether the users complaining have perchance selected None as their preferred editor which would explain the problem.
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!