Step flashing and diverter at the end isn’t working right. The last piece of step flashing above the gutter should have a bent out piece to get water running down the wall above to go into the gutter and not just run down the wall. I’d also be very suspicious the siding isn’t installed correctly there as well as it shouldn’t let water behind it.
I’d get an exteriors contractor to come and look at it, and see if they can bring the two problems together to be fixed.
No diverter is present there at all. There’s a bunch of things going on here that could be causing this problem and I think it would be best to address all of them at once:
No diverter
No drip edge present along gutter, and I’ve seen water drip from soffit vents. It’s possible water is coming into the damaged area from where the soffit meets the wall
Vinyl siding was installed over original wood composite. It’s questionable whether the step flashing is effective given the setup
Gutter potentially too small - I have to clean it out like 6 times a year even with gutter guards over most of it