Separation anxiety or normal puppy attachment?

Hello! Back again to dive deeper into possible separation anxiety? Or is this normal puppy attachment?

Our puppy is 4 months old and has been with us for about a month now. Because I work from home, I’ll try and train him to be alone by stepping in and out of the home, going to a different room to work, and generally just trying my best to let him do his thing without having me around all the time. We had gotten to the point where I stepped out for 20min and he didnt even flinch, and napped right through.

The problem is, every weekend we (my family and I) have tried to run separate errands for no more than a little over an hour. And every time he goes crazy. Its like all the training we did goes out the window and he wont even last the 20min comfortably the way he usually does. This last time, he even started scratching the door on top of howling and crying. At some point he took a short 20-30min nap before he was up and crying again.

Are we moving too fast? He is generally good at playing alone, going in his crate willingly, and us moving through different rooms without following us. So what gives? Does he just need more time/training or is this developing into something more?

Just want to help him out as best we can so he’s comfortable being alone!