Update:

I attempted to do this. Ended up having to do the purchase 3 times. I was buying an Avianca ticket with a UATP card (their version of airline credit), but SkyScanner only links to US currency purchases while the UATP card was in Colombian Pesos (COP). As a result, each time I tried the purchase in USD, it failed. The third time, I just bought the ticket from the Colombian site in COP and it worked immediately. Strangely, the Avianca CSR's were adamant that the currency didn't need to match up. Spoke to various levels of support and got the same answer. 24 hours after each purchase, the purchase was auto-canceled for failed payment. So, just for internet logging purposes, your Avianca UATP card currency MUST match the currency of the ticket purchase for it to work.

Anyway, the USD-based purchase was captured for the two times I did it - I went to PayPal, logged in, clicked the cash back link which took me to SkyScanner, searched the ticket, clicked the link that took me to Avianca's website and make the purchase using the UATP card. The first time, JoinHoney caught it instead of PayPal for some reason (fyi, PayPal deals are JoinHoney on the backend, but PayPal should have caught it, not JoinHoney). It gave me exactly 1% of the base ticket price, $2.05 of a base ticket price of $205. The $116.27 in taxes did not get awarded cash back. Funny thing is that I sent this info to JoinHoney support and their response is still "we are looking into it" even though I gave them the answer!

The second time, PayPal caught it. The math isn't really "nice". I don't know how it is calculated. I was given 503 points, which is $5.03, for the ticket. If its $5.03 on the base ticket price, then it is 2.45%. If it is on the whole ticket, then it's 1.57%. If the cashback is doubled because of their current (as of Apr 26, 2022) promotion showing 20% is now 40%, then those normal percent's are halved (1.23% and 0.79%, respectively).

I have to assume the 40% (normally 20%) cash back is on hotels or car rentals.

Even though I was given cash back, I would expect JoinHoney and PayPal to undo the cash back once they figure out the purchases didn't go through.