Gangplank farm bug?

When I play GP, I usualy farm top too get my R upgrades. But somthing wierd happends when i reach around 350-400 farm. It can go up to 500 when i just had 370 and after i take a minion or a few i lose them and go back to the true stat.

This have happend to me the first time ever since 1 patch after GPs update, and happends every game since...

Is it me or is it a bug?