Welcome to the forum.
Please tell us what your Project's specifications are. Looking in Edit>Project Settings will reveal those specs.
One could also choose the appropriate AVCHD Project, at New Project, and select "Force selected Project settings."
Also, to make sure that what you are seeing is really what PrE is editing, Rt-click on the Program Monitor (the "Timeline player window") and set Playback Quality to Highest, if it is on Automatic.
When you have Imported the AVCHD material into the Project, and dragged it to the Timeline, there should NOT be a red line above the Clip(s). If there is, then you have a Project mis-match.
Note: In version 11, the sign of a mismatch is a YELLOW line (rather than red) above the clips in Expert view.
Ah yes. Thank you for that correction. It's also confusing that PrPro uses the yellow for a slightly different situation, and still uses the red and green, like PrE used to. I started to link to a Todd Kopriva tutorial on "yellow lines" the other day, but when I read it closely, I realized that PrPro uses those to denote other things, and Todd's tutorial was ONLY for PrPro.
If I live long enough, maybe one day I can keep everything straight - until Adobe "moves the goal posts on me... "