[ic-racer]

The only things that can cause this are

1 focus shift built into the enlarger lens as it is stopped down. Get a better lens.

2 negative popping from heat. Heat absorbing glass helps a little. A Glass carrier is the cure.

3 the focus control drifts. repair as necessary

I have had cases where 11x14 and 16x20 RC paper settles a little after closing the easel. Looks nearly identical to negative popping. Easily 'cured' by a short pause before hitting the timer. I'll admit it took me a while to figure that one out. Watching the reflection of the safelight on the paper right after I closed the easel lead to the diagnosis.