Sounds like your pH meter really doens't work at high pHs, just get some strips, for the NaOH solution they are fine, for the wine, not so much.

pH stips are dirt cheap.

I always take drops and put on to pH strips instead of sticking strip into solution.

When you say pH is dropping, how much do you mean.

Your correct though if your NaOH is pure then there is no way pH can go down, only up.

You probably are at a fine pH after you added more NaOH after 11.5.