One branch limp, leaves curling please help diagnose

Hi guys, ive a big plant 6' tall,
sativa dominant
Dwc
4 weeks proper into flowering
Top of plant looks healthy, leaves are straight
Middle to bottom of plant leaves are curling
Res temp is good, between 18-25 with 2 bubblers inside
Ph is 5.8 last measured
Ppm is v low at 300 as the plant suffered from a nute lockout at 1300ppm and almost died but was fortunate to survive. Has been 2 weeks on 200-300ppm. Yest i upped the ppm to 400 and one branch gone limp. I quickly changed out the res today and topped with ph 5.8 water n ppm 200. Branch still limp after 6 hours now heres a pic pls let me know what i should do to keep my baby strong.
Any help is appreciated!!! Thanks guys

Individual branches going limp makes me think of either damage to the branch or root rot. Are your roots white and healthy?

yes roots are white and healthy. and branch is not damaged.

Any input guys... Btw just recovering from nute lockout what should i be doing? Im going really low at 300ppm they are eating the nutes maybe 100 every 2 days or so.. So i just top up phed water n adjust ph but keeping nutes at 300. At what point do i up it? Cos many leaves are still curling, even some new leaves

It looks like nitrogen toxicity, which doesn't make sense at that low of a ppm.

Is it getting enough light? It looks really spindly, like it's reaching for light.

I wouldn't up the ppm yet with the leaves looking curled like that.

Limp branch.. no idea what that is from. Possibly not very good genes on this plant.

I would think about getting some better genetics, possibly better lighting too.

specific branch issues, I always suspect mold, look at the stem to make SURE that they are no grey or black blobs of mold, it only take s a SPECK to make the whole branch do that, other than snapped branches or mold I've never seen branch-specific problems, so I highly suspect either of those.

Looks like more Nitrogen then she wanted. Dark & Clawing.

Look at post 86 of my last grow. Did the same thing on an "unknown" strain from a breeder. Cut way back on the N and she turned out excellent.