I'm having the same issue. I haven't been able to find any commonality with them either.

this is usually related to the end user resetting the network settings on the device. Unavoidable unless you use global proxy, which isn't favourable in some environments.

In our situation it is not the user resetting the network settings. The WiFi encryption key is still working which would not be if they reset network settings. Additionally we cannot set new proxy settings manually. We set the proxy manually and then close out and go back in and they are gone. Like they were never saved. Our only option has been to un-scope device from the profile and then re-scope it. This happens so often lately that we just made an extension attribute called WiFi Profile Installed that is either "Yes" or "No". The profile is scoped to all students except those in the smart group based on WiFi Profile Installed = No. Now we just have to set that attribute to No and then back to Yes and the profile is removed and then reinstalled. I should point out the iPad connects to an open network with limited access once the desired profile is removed. This allows the profile to be reinstalled after removal.

What you could do is send “shell wi-fi” profile down to the devices that includes the ssid, any relevant certs as well as the proxy. I don’t like that method but have to use it on our 802.1X network. In our case we want the WiFi to have more than one trusted cert, but wish for the users to populate their own creds. With a profile, they wouldn’t be able to alter the proxy, but you would lose the ability to “forget” that particular WiFi network.

Hi guys,
I'm facing this issue on DEP enrolled devices way too often with about 250+ devices getting really frustrating. Is there a permanent solution yet?

We're seeing this too. 3000+ devices, all in DEP. Had occasional issues, but seems to be getting worse. In some instances, the user can reenable the auto proxy on the devices, other times it won't let them save at all.