I dont think thats normal unless quality control is getting more lazy or maybe the parts are more inferior.
My family and I all have owned all kinds of air cooled mowers and not one of them has done this. I also worked for 2 lawn companies when i was 19/ 20 years old. They both had gravely mowers, and they never burned oil at anytime.
Gravely Pro Turn 52
Gravely Compact Pro 34
Toro 21 super Recycler Mower
Husqvarna 25cc & 28cc 2-Cycle Trimmers
Shindaiwa EB802RT Blower
6.5 x 14 Top Hat Open trailer
1966 International Harvester 404 Tractor
72" FramTrac grooming mower