It's a Phosphorus Deficiency.
I'd wager that it's the result of too little P to handle that many flowers. I can't tell from the picture if you've got the suckers all pruned. (Suckers are the shoots that grow just above the terminal (sun) leaflets).
I would prune that gal if I were in your shoes. I'd take the main stems down to 1 or 2, and be sure all the suckers were lopped. Ime, you'll still get more, better fruit off of 1 or 2 than the 4 you have, because the nutrient/water uptake will be more focused.
This is just anecdotal, but with the fruits/veggies I've grown where there's debate over whether it's better to have more flowering stems or one stem with more flowers, I invariably have better performance with the latter.
Here's a good pruning guide, and it applies well even if you're not as concerned with space as they are (that's why I might keeping 2 main stems instead of just 1): https://www.youtube.com/watch?v=mPf7a96eOlQ
For feeding, the retail tomato CRF (9-4-12) CRF just doesn't have enough P to meet the demands of a heavy-flowering tomato plant. Switching to a heavier P fertilizer in late season is necessary, or you get so-so fruit and purpling.
I usually use a 10-10-10 CRF, with lime and epsom mixed into the medium and some epsom spray later in the year - but Miracle Gro shake and feed "For Palms" is an 8-8-8 with micronutrients built-in, and it's a lot less work.
I don't see anything you've written about what you've fed it. If you haven't fed at all, relying on the fertilizer in the potting mix, that's almost certainly the issue.
Don't get me wrong, Miracle Gro, for as much flak as they get from some folks, does make good products. It's not that the MG mix is bad, it's that tomatoes are heavy feeders.
My recommendation: to keep it simple, get the 8-8-8 Miracle Gro Shake 'n' Feed from Amazon. (I've never seen it in box stores, but have at nurseries. It's price there is usually inflated.)
If you don't want to lose a week before the feeding (shipping), order the CRF and pick up a small package / vial of liquid soluable fertilizer at the store to tide you over. It's a good thing to have on hand, even if you're only going to use it once on the beefsteak.
The 8-8-8 is a great all-around fertilizer for fruits and veggies. The MG "all-purpose" is better for decorative plants with moderate flowering.