The following is from the Qatar website
"If your journey includes travel on an airline other than Qatar Airways, including a Qatar Airways codeshare flight operated by another airline, that operating airline may offer you a lower baggage allowance. You are advised to check with that airline on their allowance."
The following is from the Bangkok Airways website
"Economy Class : Maximum weight allowance is 20 kilograms for adult and child / 10 kilograms for infant."
I think you are stuck with the lower limit unless Bangkok Airways advise you differently.
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