I did a quick search for the electrode listed and found very little information. I'm not even sure that the electrode I found is the same as that being used in this inquiry. The Stulz Sickles website doesn't list the same electrode listed in the inquiry.
Based on what I did find, it may be the same electrode, it appears that the deposit is austenitic, which would explain why the weld doesn't crack. However, the high manganese and chrome could diffuse into the mushy zone of the HAZ on the carbon steel side of the joint. The increased alloy content in the mushy zone could result in martensite on the mild steel side of the joint. The hard martensite would be sensitive to diffusible hydrogen and result in delayed cracking if the electrode used is not of the low hydrogen type or if it is low hydrogen, it isn't properly stored in an electrode oven to prevent the absorption of moisture.
The delay in the occurrence of the cracks sounds like delay cold cracking that can be caused by hydrogen from any number of sources. So, it would be advisable to make sure there is no grease, paint, moisture, rust, or any other sources of hydrogen in or near the weld joint.
I would give strong consideration to preheating the carbon steel before welding to it and I would check with the manufacturer to see what the recommended storage conditions for the electrodes are. If they recommend storing the electrodes at temperature, I would definitely obtain an electrode oven. I would also observe any exposure limits recommended by the manufacturer.
Let us know what recommendations the manufacturer provides and whether you have success or additional problems.
Best regards - Al