I’d like to do a series on problems all of us have from time to time when growing tomatoes. This week I answered a question about catfacing. (Please don’t run through the streets screaming in horror. Franken tomato happens to most of us at one time or another! )Below is an excellent article on possible causes. I am sharing it here. Click on the link below to read his/her take on catfacing. Thanks to The Tomato Bible.
https://www.tomatobible.com/tomato-catfacing/
Pretty ugly, right? There are multiple reasons that this could happen.
For one, many of the largest heirlooms are susceptible to this. You would never see this in retail. They want pretty, round, moderate-sized tomatoes.
A big reason could be the weather. Wide fluctuations of hot, cold, and warm, interfere with the flowers as they develop.
Another reason could be your watering practices. Inconsistent watering, too much, too little. I know it contributes to cracking on thin-skinned tomatoes.
Weather, we can’t control. The varieties we choose are something we can change, as well as how and when we water.
The good news? You will lose a lot of tomato when you trim the icky spots but the tomato is still very, very delicious! Hands down, they taste a lot better than the perfect, plastic ones at the store.
Just know that is isn’t usually something you did.
Notes From The Tomato Lady 
For ours, it was supposedly a combination of stimuli, but particularly the weather stimulating a reaction to a minor nutrient deficiency. One of the common varieties that performed well in a former garden did it almost annually here; so our solution was to simply not grow that variety. I am not in the habit of trying new and interesting varieties, but do not mind not growing what does not like growing here. Now I can not remember which one it was. I do not grow any of the humongous sorts.