So I'm having a similar issue I finished the collection and was rewarded with the jack Russell. Well it never showed the dog in the pen once I placed it in my shop. I thought it was weird but I ignored it. I was even allowed to cross breed the Jack with my Corgi. I had tried putting the pen back in my inventory and then putting it back in the shop floor to see if the Jack would appear (because now it wouldn't let me collect from it). Well once my cross breeding was done I went to place the CoJack and the Jack Russell pen was completely gone. I looked in my inventory and it isn't there, and I know I didn't sell it... I just wanted to get a Jack Russell so I could continue to crossbreed!