I reached out to fellow panelists and received this explanation. Hope it helps:
The triangular stems are probably fasciation, a malformation of stems, flowers, or fruits that is typically aesthetic only and rarely affects the health of the plant. Just a weird plant mutation. https://www.canr.msu.edu/news/plants_with_abnormal_growths_the_interesting_phenomenon_of_plant_fasciation As for the more cactus-like growth, this may be caused by seeds from another plant sprouting in the potting media (this seed may have come from a neighboring cactus in the greenhouse). Sometimes I have found several Schlumbergera in an original nursery pot when I go to repot it and this seed may have come in with one of these plants. Or this plant may have been hybridized with another cactus and this growth is from that parent. Hard to say exactly. If this cactus-like growth pulls out of the soil easily, I'd say it is the former. https://www.schlumbergera.net/hybridization/
Here is another response from a fellow panelist - good information: The "more cactus like growth" is very common on older Christmas or Easter cactus leaves. The leaves gradually age and the cuticle toughens and looks woodier. If one takes cuttings from the coarser looking stems they will put out greener, younger looking growth. My Christmas cactus is about 105-120 years old and it has a similar appearance. It was my Grandmother's.
This work is supported in part by New Technologies for Ag Extension grant no. 2020-41595-30123 from the USDA National Institute of Food and Agriculture. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.