Put a reverse curl in the film end before you wind the last bit onto the reel. That should straighten it up. And make sure to wind the end of the film fully into the reel. If you leave the end in the loading cutout it will have more scope to curl.
I can't say I have ever noticed a problem like this. The film surface on the inside is going to be the backing, not the emulsion, if you load conventionally with the emulsion in, and that should not be sensitive to local flow differences in the outer layer. 1500 or 2500 reels, and what rotation speed?