@m0use it's middle of plant, looks like calcium deficiency starting.
Why could be many reasons , solution only one ;
Good wet dry regime
Good pH
2-3 waterings just with pure water no additives
0.5 or 1 ml / l of CalMag depending on situation when you finished pure water waterings and she is drinking regularly . I tent to guess that's it's not lack of it, but problems occurred from not following above mentioned requirements ...
All the Best !