Sounds like evaporator was built wrong-- seen it twice in my life- you have done all the right things- and tested correctly- so pull out evaporator and study the design, there are tabs that show the "blanks" that reverse the flow direction- should be 5 sections south, 5 sections north, 5 sections south- with two "blanks" between direction changes- Chances are good your tabs are on same end, (reversed) built by wrong by goofy assembler....probably texting while assembling....
Last one I saw was probably 20 yrs ago-- I could hold it up and see right through it- inlet to outlet....
'95 Chevy 3500 454 Dually 142,000 miles
'94 Chevy Astro Van 358,000 miles!
'86 Buick Park Ave 192,000 miles
'86 GMC S-15 Pick-up 150,000 miles
'92 Chevy Lumina Van - 265,000 miles- given to me to see how far it can go! all above are white!