Two like causes come to mind.
1- Trim. if the trim is slightly outboard of center, the chain can hang and begin a tentative shift and at high load complete it. Also high loads cause frame deflections which can very slightly change the trim. If you're at the edge, this can be enough to cause a shift. Try adjusting the trim inboard a bit, but not enough that upshifts aren't clean and crisp.
2- wear, you didn't say how many miles, but worn chains and sprockets don't mesh perfectly, and high loads can pull the chain forward on the sprocket which can cause the engagement to hang. You can get a hint of how likely this be measuring the chain for stretch.
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