Mechanism for Amino-Imidazole Ring Fission
I'm afraid the answer to how this ring fission occurs is... it doesn't, you just made some mistake.

Here and here the authors used the same reaction as you and got the product in good yields. I think you might not realized that the product is a very hydrophilic amine (or salt, depending on workup). If you did an extraction of your reaction mixture then the product surely ended up in the aqueous phase.