What could trigger low *data* mode on a iPhone besides the manual switch?
In regards to my comment, triggering off low data mode for Wifi.

Wifi Low Data Mode

To turn off WiFi low data mode regarding your Wifi connection, instead of cellular low data mode. This should not interfere when you change your cellular connection to low data mode.

Step 1: Open Settings and go to Wi-Fi.
Step 2: Tap the name of your WiFi network.
Step 3: Enable or Disable Low Data Mode

Suggestions: Update to the latest operating system, as well.

You might also notice that sometimes applications will not be allowed to refresh their data in the background, and FaceTime calls as an example are not the same quality as they normally would be. The same goes for streaming video, as well.

Enable Or Disable Low Data Mode For Celluar


Step 1: Open the Settings app and go to Cellular Data &gt; Cellular Data Options.

Step 2: Enable or Disable Low Data Mode.

Step 3: Might be Celluar Roaming