Internal white tissue. It's one of the physiological fruit disorders. Linked to high temps and/or low K during ripening.

If you continue to have problems with this, try picking tomatoes right at very first blush and ripening them inside on the counter.

You're the best! Always so helpful! Should I be adding potassium?

If you're fertilizing regularly with a balanced tomato fertilizer, I would stick with that. This issue probably happened before you started fertilizing regularly. In a container environment, it's very easy to overcorrect and create more imbalances when you start to fiddle with single nutrients.