Nute burn it looked like from the tips. Ide say both gnats and nutes since You backed light, that didn't help so it's not that.
Since flushing you increased the gnat problem.
I would let dry 40 to 50% then feed half strength and check runodf. Immediately increase fan near tops of soil to keep it dry and possibly adding layer of vermiculite or something to cover the soil.
I would check runoff to see leaves of PPMs and PH. If I had to guess it started nute burn, then flushing increased gnats and now you got that problem to fix.
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