My transition (iris round) loops (instead of simply prolonging)

Hello,

I recently encountered the problem that I can no longer use the Iris (round) effect correctly as a transition between two videos or between a video and a static image. This is because the effect always lasts less than a second – if I extend the transition, the effect keeps repeating itself and keeps showing image A --> B, A --> B until the duration of the effect is complete, instead of making a long, smooth transition from A to B. Other transitions work correctly here, but the Iris effects do not. There is enough image material, I don't get an error message that frames need to be filled in, or anything like that, and, as already mentioned, it works fine with other effects.
What am I doing wrong?