First of all, a lot of different devices come b under the umbrella of imu.
To my understanding most imu sensorsensors report angular rates. So he'sit's very common in the scenario you have submitted to get 90 degrees instead of 180. ( keep in mind that they report degrees per second or similar units).
There are of course higher grade imu's that combine several sel sensors(accelerometers, gyroscopes, and magnetometers). These are able to provide absolute angular measurements. However, they are costly.
There are other contributors with more experience in aviation here that might give a more illuminating insight.
So bottom line; IMHO it depends on the modelimplementation of the imu makesensor.