Nobody would have any insider information on exactly how those agreements with partners are. However there are enough information in the 10Q and 10K to give you a better idea on how such agreements might work, instead of "I'd think".
You dont need to know why, you just need to realize such expenditures are incurred and are reported in the 10Q and 10K as required by SEC.
If you so incline, you could dig thru Chase 10Q and 10K to see how much it pays for the partner currencies. The disclosures typically are listed in the Notes of said reports filed with SEC.
As a starter, this is how much United recorded its side of the miles purchased by Chase in UAL's 2013 10K. Note the purchases mentioned are for 2014, 2015, 2016 and 2017 - several years in the future at that time. As mentioned, it is recorded as a Liabilities on the partner's side, an obligation to Chase. Likewise, it is recorded as an Asset on Chase side - it has PAID for such currency and carried them on its book - it will be expensed over time when the miles are awarded to various earning activities.
https://www.marketvis.io/stock/ual/f...osuretextblock
Not saying it would have same agreements with hotel loyalty programs, but the likelihood is high that Chase has similar agreements in place with their partners, as that is pretty much a standard practice between banks and their partners of the cobranded cards.