The question has already been answered on the official docs (Under Spaces):
https://www.digitalocean.com/docs/accounts/billing/bandwidth/

“Traffic from Droplets to Spaces does not count against your Spaces transfer allowance (because inbound bandwidth to Spaces is free), but does currently count against your Droplets’ outbound transfer allowance. We’re working on changing this behavior.”

However, since the docs explicitly stated that the behavior will be changed, are there any updates on this issue?