Distinguish between hardware or firmware issue
Here are some rules of thumb:


It's usually a firmware problem.
Intermittent hardware failures can often be made less intermittent by changing the temperature, voltage, clock frequency, or other operating conditions. (Some firmware errors are like this, if their behavior depends on the state of the hardware.)
Gross hardware failures are usually pretty straightforward. If something basic is broken, and replacing the unit fixes it, it was a hardware failure. 
Know the hardware and the code. A good knowledge of the interacting parts of the system is vital.
Sometimes a firmware error can look like a hardware problem (e.g. timing violations). Adding delays in your firmware or lowering the CPU frequency can sometimes help pinpoint the error.
Try to narrow down the exact subsystem that's failing. If it's an internal module, make sure your register reads and writes are working properly. If you're communicating over a serial bus, check the bus with an oscilloscope.
If your firmware produces a bad decimal value, convert it to hex to look for flipped bits and uninitialized bytes.


In your specific case, it looks like one bit in the most-significant byte is getting flipped (0x08000000). This could certainly be a hardware error, but to prove it you'll need to verify that you're talking to the module correctly. I'd suggest first checking the datasheet to make sure you've configured the RTC properly and aren't violating any constraints. If you have another CC3220, see if you can reproduce the failure on it. If not, try slowing down the clock to see if the failure is timing-related. If you can tweak the supply voltages, try running at the min, max, and nominal voltages on each supply and see if the failure rate differs. Looking at the source code for sl_DeviceGet() could also be helpful.

If you try reading the RTC as fast as possible, do you see many failures in a row (which could mean a brief period of hardware failure), or are they more spread out?