You have a balanced coin. In your first 350 flips, you have obtained 300 tails and 50 heads. Which has a higher probability of coming up on your next flip: heads or tails?
Assuming it is an unbiased coin, both heads and tails are equally probable. (The fact that you declared this to be a balanced coin implies that the coin is unbiased).
Long runs occur which do not match expected outcomes but this does not invalidate the underlying probability.