A two-team teaser with a TIE and a LOSS should be graded as a LOSING BET (despite illogical "old school" practices of the past).
The logic is VERY simple:
If one game of the teaser is a winner and the other game is a loser, the result of WIN + LOSS is always LOSING TEASER BET.
Now, if on the "WIN" game, your team does slightly worse, and merely TIES instead of WINS, why should the net result of the teaser be better? A (TIE + LOSS) is WORSE than (WIN + LOSS). So, why on Earth would anybody expect to get a better payout on a worse result?
Though some old school places did this, it was just plain wrong. Maybe, they used it like an incentive or a rebate program to keep players happy, but it was still an illogical policy.
There are still places out there with players getting 14/6 instead of 13/5 odds on two-team parlays. It's because they've managed to grab some old-school players that don't know they are being ripped off. The players' ignorance does not make 14/6 the correct odds (nor does it make TIE+LOSS a push).