2015 Sonata, SE, ECO to SPORT mode shift issues!

Just got a used, certified, pre-owned,  2015 , Hyundai, Sonata SE, 32-K
miles, nice car, I really like it, except for this!  I've never driven a 'dual
transmission' vehicle before.  While driving at 35 mph, in automatic mode, I
moved the gear shift lever into the manual mode/gate and have an immediate
'deceleration', car slows and almost dies, pressed on accelerator and it got
worse-moved gear shift lever back into automatic mode and it stops?  What
am I doing wrong or is their something else going on?  Do you have to hit the
'(sport/manual) mode button before switching from the (ECO/ Automatic)
mode?  The dealer never mentioned it!  Or is that normal for the RPM's it was
at, at 35 mph when I switched modes?  It happens when ever I switch
modes.  I've only driven the car a few times since purchase!  Dealer and
mechanics sound like they don't have a clue when I called to explain it to
them!  The owners manual isn't all that clear or mention it either!  I'm looking
for an answer from anyone who has experienced this too or what I may be
doing wrong not advice to take it to a dealer if I can avoid that! (no car fax
report was provided at purchase!)  HELP?  Thank you!