Dropped valves on the GY6 on a new engine aren't related to heat. It would be an out of spec guide from the factory, a defective head. This is the second time I've dealt with this. The first was on my brand new lime green Yerf Dog GX150 over a decade ago. If the guide is undersized, it will pop out of place after the first few heat cycles. I believe you've encountered the same thing. It's not due to overheating.
Heat issues on GY6's come down to overly lean jetting which wouldn't be covered under warranty. And - please believe me. I'm advising against what's most convenient for me by saying it's not a heat issue. Other places would just go with the overheat idea and try to blame it on your jetting or something else you're doing. I think the problem is in the part itself, and I'd like to handle it under warranty.
Jason, since you're already inside the engine I think the best course of action is to ship out a new head to you. Are you Ok with performing the reassembly? I have two guides that'll help with the process:
Easy GY6 Cam Timing Guide
https://www.buggydepot.com/tech-center/ ... enter-tdc/
GY6 Valve Adjustment Guide
https://www.buggydepot.com/tech-center/ ... y6-valves/