Last night after posting, I did a little reading on the web about mower deck airflow. In addition to learning about some interesting patents, I found a lot of info on clippings buildup causing some issues.
I had the deck flipped up last night to remove the blades to check balance. I saw a little buildup, but nothing I haven't seen before. I can scrape it off, but it's maybe an inch thick in the worst spot. It's more like a thin coating in places, no major clogs. I'll clean it out, but I'm not thinking this is the problem...it can't hurt to take care of it either way.
It sort of makes sense that the mulching blades that are designed for lift would need to move some air, and maybe I'm suctioning them down. I should probably just cut the grass a little higher, lol.