Branch die back on P. Sylvestris

So I posted a thread about this tree a few months back. I was really happy with its first styling. Over the summer I didn’t turn it and the back branch died back from behind shaded out. However today I noticed the whole dirt branch is showing signs of weakness and possible eventual death as well. It was not shaded by any means and the rest of the tree is very healthy. I also have come to the conclusion this afternoon that I have a colony of ants living somewhere in the pot. I’m wondering if maybe the ants could be causing the single branch to die? Drainage is great, watering has been on point, aside from the back branch everything is good to go. I’m pissed because the loss of the back branch really screwed this tree up. Like I said I was happy with its first styling. If this first branch goes, which I’m pretty sure is inevitable if I’m seeing discolouration and wilting like I am, I won’t have much of anything to work with anymore. Any suggestions? If I take this outa the pot to inspect the ant colonization and have to do major work I fear it won’t make it through winter. I’m in a tough spot and could really use some suggestions or a helpful hand.