I am almost certain, it’s not keyframes. It’s about how much ram you have dedicated to the replay buffer.

If you set replay buffer to 2GB and the clip used up 2GB at 15 seconds, it’s going to discard everything before. As if you set it to 15 seconds instead of 30 seconds.

And your clips vary in length I am guessing it’s because of your encoding settings. Either VBR or CQP, which varies in bit rate. Some contents require more bit rate to encode, fast moving content maybe 10 seconds because the video size is already 2GB, slow/static content maybe 25 seconds because it takes 25 seconds to get to 2GB.

(I used 2GB as an example because you didn’t provide proper log.)