Sounds weird. As mentioned, AC powerheads will spin in either direction when plugged in and with the original impeller, it doesn't matter, since it's not directional, it pushes water through the volute either way. As noted, the tab is there to try and bounce it into the proper direction when fitted with a propeller.
It is possible that the propeller may be fitted on to the shaft the wrong way around though. When you look at the prop from the side, you should notice that the blade tines are sort of flat on one side and curved, or rounded on the opposite side. If I'm not mistaken, the flat part should be at the front, away from the powerhead body and the curved part at the back. Don't know if that description makes sense. If not, I'll try to come up with a pic or something.
Edit. Photos attached where the pen represents the driveshaft. Correct, Incorrect, and a moded Aquaclear impeller with prop in correct config.
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