Is your R2 bucket connected to a sub-domain within your own domain?
How often you’re clearing the cache for your zone?
Clearing, purging the cache for a domain example.com includes all sub-domains as well which are proxied 
, therefrom the whatever was cached from a sub-domain sub.example.com connected to your R2 bucket, was cleared and needed to be re-cached again.
Furthermore, consider setting “Browser Cache TTL” to e.g. 6 months and “Edge Cache TTL” to a month at least for the R2 connected (sub)domain.
You’ll need to follow these directions here: