2000 blazer acts like trans slips from a stop 0-5mph only
 

Got a strange tech question. My 2000 from a stop usually 0-10 mph acts like its slipping from a start. Once it gets going its fine through all gears. I have HP tuners and have commanded 1, 2, 3, 4 from a stop it does it in every gear as well as reverse. When datalogging slowing down for a turn/traffic without stopping ~15mph, I can see the transmission downshifting gears to first it won't do it at all when starting to accelerate again. I've commanded first and second gear all the way to 6k rpm from a stop and it holds power and gains mph. It makes me think it has a line pressure issue but I would think it would slip in higher rpm/power. I'm not 100% sure the trans is bad. Anyone had a similar issue?

just did some investigating earlier. dropped the pan and found the filter laying in the pan. never in my life did I expect to find that. put new filter on and fluid she's good as new. I'm confused as to why once it got moving without a filter it never slipped?