good eye Doctor Gorlax!!!

this is a good example of why it can't hurt to pull ones plant out, if you can't figure anything else out, roots and soil give good clues....also why too big a pot, often can mask the troubles,...too small a pot has its own set of troubles, but they are much easier to rememedy....
this can happen with perfectly good soil over time as well...organics get broken down, particles get smaller, water starts to "sift and "strain" them through to the bottom of the pot, if roots or holes are plugged, you get this issue, if holes are open, sometimes the reverse happens, larger and larger channels open up, and soil is washed out....(this is a better choice, but ou wonder why root ball won't get wet)
Plants put into Better growth situations practically GlOW with good health....pretty amazing.....