Well first off, I dont like to flex hard all that often because for me it does seem to impact recovery if done in excess. (no science there, just how I feel)
However, the fact that one arm is failing before the other while doing db curls is most likely just a muscle imbalance. If you had been doing bb curls for a long time your one arm was probably doing a little more of the work than the other. Keep doing the unilateral curls until things seem a bit more even between your two arms. Also, begin your reps with your weaker arm and once that arm fails do not continue to do more reps with the other arm. Give the weaker one a chance to catch up and correct the imbalance.
