Good Morning H.. I do believe it's a pretty normal occurrence. Our Gr son same age as your little guy, doesn't like the big potty not either for poopoo, the water Splashes his hinny. ;) Now he likes the auto flushes once he figured out he could just stand there and play with the silly thing. Moving his hand back and forth across the beam.
Corbin will poopoo in his pull up at nap time, he starts pre-school also soon. And for a while he did very well on the big potty for poopoo, but Now Nada chance. He will go pp though alone. He will be gone from 8:30 - 11:30 so I know his routine enough to know he will not go poopoo while in pre-school. Of Course he will probably prove me w-r-o-n-g :)
About those auto flushes, you could have dad take him when out and about and cover the beam with his hand, then let the little guy tell him when to move his hand. Make it a game in a way. Usually little boys want to be big boys with daddy, so if daddy lets him know before hand he is going to help him it might work.
Let him keep His potty chair for a little while longer, just let him know when he goes to Big boy school they have big pottys there and he will need to use those like the other children.
Be Patient he will get back to the program soon.
God Bless
K. Nana of 5