3 years ago I purchased a dwarf containerised apple tree with 2 varieties on 1 stem (Cox’s pippin and James Grieve). Year 1 produced 25 fruits. It was repotted after that season and produced only 2 fruit last year and 1 this year. It is outside in sheltered patio and has been since purchased. Pot size is 45cm diameter. It is well watered and fed. Garden is south facing.
There is an outside possibility of vine weevil damage to the roots, in particular if no ordinary garden soil was mixed with the compost at potting up. Apple scab disease of the foliage might well have weakened the tree too.
Apple trees are always a bit tricky in pots … possible but needing close attention.