That's actually the default behavior of all text editors that use the standard NSDocument architecture of OS X, like Apple's TextEdit or Pages and other apps like iA Writer.

Here is an Apple support document that explains this feature in detail: https://support.apple.com/en-us/HT203659

This is what Apple suggests to disable this feature:

On Macs, you can use either of these methods to stop the app from saving your document to iCloud:

• Turn off Documents & Data: Choose Apple () menu > System Preferences, click iCloud, then deselect the Documents & Data checkbox. When you turn off Documents & Data, the iCloud Document Library no longer opens for any Mac apps that use Documents in the Cloud. You can continue to upload and download documents using the iWork for iCloud apps at iCloud.com.
• Save the document to your Mac. Your edits will no longer be saved to iCloud, and your previously autosaved document will be removed from iCloud.

iCloud sync on iOS 8 still does have occasional hiccups - not just in Textastic, but also in other apps including Apple's. When I had problems like this, it often helped to reboot the device. It looks like the iCloud sync process sometimes gets stuck and a reboot restarts it.

It can also help to edit a file that didn't upload as this triggers another upload.

This *should* all work automatically, but unfortunately, sometimes it doesn't.

Can you please give me example files that I can use to reproduce it on my device?

Can you tell me which method you use to switch between tabs? Do you use a shortcut or do you click on the tab?

I tried the following:

• Hit Cmd-N to create a new file
• Type text into the file
• Hit Cmd-N to create another new file in a second tab
• Type text
• Hit Cmd-Z -> this deletes the contents in the second tab, but doesn't delete text in the first tab

It's great to hear that you could figure out the problem.

Please try to enable the "Debug Log" setting in the connection's settings screen. Tap on the (i) button on the right side of a connection to change its settings. This creates a log file when you try to connect to the server which might help you find out what's wrong.

Thanks for the detailed explanation and gifs - that's a good point. I've added it to my todo list.