if i were growing a tree in a pot ... i would use a much different media.. than what i think i see in your pot ... and its not easy in these pix to differentiate which leaves and pots go with which plants ....

in other words.. you may have a watering issue .... or more specifically .. a drainage issue ... too much water being retained ...

ken

ps: i would be disturbed by the color of your house.. if it werent basically the color of my living room ... lol ...

It looks as if the tree has already flowered and has leaves. Is that the normal timing where you are? It seems very early.

You might look at fireblight. https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=19122

I can't tell from the photos but is it just one grafted branch that is affected or all of them? On multi-graft apples, it is not uncommon for a graft failure, In fact, I:d pretty much expect it :-) The other question I have is are all the gaffed cultivars well-suited to a SoCal climate? In general, most apple cultivars are tricky to grow in such a winter mild climate due to insufficient chill hours and that can certainly cause stress and poor performance.

Issues with fire blight may be very relevant as well, as various apple cultivars will demonstrate various degrees of resistance/susceptibility.