This is by design.
When gifting points from a Post Checkin Campaign, it's apt to use the {{{campaign_name}}} in a sense of "You received X points from {{{campaign_name}}}" for example.
The "Reward_reason" tag will work as expected when gifting a redeemable, however.
As this is expected functionality, any requests for updates to this functionality should be passed along to your Customer Success Manager.