I had what looked exactly the same on a younger plant ... Yellow Brandywine ... while it was still in a 4-inch pot earlier this spring.
I stripped off the yellow foliage, transplanted it up to a larger pot in fresh mix (I realize that may not be an option for you now), and gave it a shot of MG tomato food. It recouped in less than a week and now that it's been out in a raised bed for about a month, it's lookin' great. Not as healthy as the regular Brandywine, the Brandy Boy F3, or especially the Earl's Faux (that dang thing's a monster), but no signs of whatever the ailment was. It didn't set fruit in its first two blossom clusters, but otherwise it's healthy.
Good luck,
PV
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