The need for flushing is dependent of a few things, the grow media and most importantly the type of fertilizer used, If it's an organic, flushing shouldn't be necessary, however, if using synthetic nutes it is common to have a build-up of "salts" from the chelated ferts. These will eventually build up and cause lock outs. Flushing at the beginning of flowering is pretty common with synthetic nutes. The plant certainly shows symptoms of a root issue, probably from too much water, which damaged the root, and now the roots can't absorb enough water (or nutes). Are your pots sitting directly in the trays? They should be raised up on blocks, so the pots are not sitting in runoff water.