What is wrong here?

probably related to irrigation habits (pH). looks like you aren't getting the entire substrate wet and letting it dryback a bit before irrigating again. the creates irregularities in nute concnetration for pockets or concentric zones around what does regularly get wet, which can cause swings in pH. small plant, big pot -- sometimes have to irrigate a smaller area, but need to becareful of how many times you do it as each time it dries back where there are no existing roots the concentration can just keeps building up. it dries and becomes a solid again but will dissolve once you irrigate later on... always increasing area that you irrigate around a small plant as you go will help avoid this... a column all the way down (some runoff ensures it) to promote proper root growth and limit superficial roots. even if in soil, you want "some" runoff, but less than soilless so as not to leech out all the good stuff. With a pH issue, you want a more runoff until it corrects itself. If in soilless, 10% runoff is fine. In this case i'd amp it up to 25-33% in soilless until pH drops back down. pH test your mixed solution or any water that is dumped on pot. can check runoff or a soil slurry to see if it is drastically different -- this is not a precise pH measurement for the substrate... but can help be more certain of what is happening. one time use of sodium bicarbonate to raise pH is no problem. don't use this for 3-4 months, find a different alternative.. don't pay the obscene price for "ph-up or pH down" products. white distilled vinegar will work for and ph-down you need. measure it out and you only have to spot-check in future. as long as tap ph doesn't fluctuate or your formula of nutes mixed in... the latter would require a different dose to correct pH. get entire substrate wet... unless it's in a huge pot for plant size. get some runoff... if pH, this should bringn it back into a safe range ... will take time to see results after that.