The cash back amount is based on the transaction amount, so it should include the 1.25% conversion fee.
If it's not listed on the first page, then no one has tried it. You can be the first to try.

Spotify is billed in RM. Fixed at RM 22.40 every month.

Because it's a foreign merchant and billed currency in RM, CIMB charges a 1% cross border fees while PBB doesn't. Calc as below.

CIMB: RM 22.62 * 5% discount = RM 21.49
PBB: RM 22.40 * 5% discount = RM 21.28

It's negligible. But I keep my spotify charged in my CIMB because there's so much more that I could charge to PBB rebate wise that CIMB can't (dining, airfare, insurance etc)

no issue with starbucks online reload using pbb vs. gotten 6% cb.

for quantum's 5% cb, should be no issue. just try and update us here 

That if the transaction is in foreign currency. But Spotify is already localised and charges in RM.