Hi think that’s because you have 2 very different strains so they are crossing and dominant genes are winning - maybe these are better adapted to your area?
My view is it’s best to keep one strain and keep selecting from that. It’s difficult unless you live in an isolated area or have good persuasive powers and can influence all beekeepers in your immediate area to do the same thing! But selecting from your best and keeping to a strain without too much variation, is better than doing nothing & you can make easy progress on temper & reasonable (rather than huge) productivity