Two questions - potty training timing and submissive behavior

Good morning!

I'm still a new dog owner and have a few questions. I have a 16 week old soft-coated wheaten terrier and he has been with us for just over a month. Up to that point, he was with his mom and other pups at the breeders.

He is doing great with potty-training (99% of the time) and knows where is supposed to go and when. We take him out about every hour and if he needs to go before that, he will whine at the door or ring his bell. BUT, every once in a while he will just "go" on the floor (maybe once every 3 days), with seemingly no warning. Is this anything to be concerned about? Or is it just that he is still a little guy and needs time to get to 100%? We do use the enzyme cleaner after every accident.

My other question is a personality question. He seems VERY VERY submissive. We socialize him frequently with other dogs, at the park and in friends' homes. Without fail, upon meeting any new dog (or cat, for that matter), he gets low to the ground, tail down and then flips on his back. It's kind of funny when he does it with a cat or tiny chihuahua, but should we be concerned that he is too submissive? He isn't overly fearful in general but just seems to lack any confidence when dealing with other animals.

Thanks in advance for your input!