Search.lookupFields returns differently structured outputs in different environments.

In one of our scripts, search.lookupFields returns different types of output depending on the environment.

Example

SB : {"custbody_ossm_dummy_asset":[{"value":"4490","text":"1 : BW Flex"},{"value":"4491","text":"2 : BW Flex"},{"value":"4492","text":"3 : BW Flex"},{"value":"4493","text":"4 : BW Flex"},{"value":"4494","text":"5 : BW Flex"},{"value":"4495","text":"6 : BW Flex"}]}

Production : {"custbody_ossm_dummy_asset":{"value":"18835,18836","text":"KA425-0027608 : BW GasAlertMicroClip XL LEL(F) O2 H2S CO (Yellow, UK),KA425-0027805 : BW GasAlertMicroClip XL LEL(F) O2 H2S CO (Yellow, UK)"}}

Does anyone know why the output structure might mismatch between Sandbox and Production?
Please note that I've already confirmed the field configuration is identical in both environments.