Hmm, I just followed your steps:
I tamed the cattern, the chest lock disappeared. I confirmed I could open the chest, but also reloaded and tried not touching the chest at first, same result.
I went to bed, when I woke up the next morning, there was an egg, I collected it. The lock still didin't appear.
I opened the chest with no issues.
Was the lock visible on the chest? And what did it say when you tried to open it?