test_derive_blfl fixes

[rammprasad]

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[github-actions]

Package	Line Rate	Health
sdtm.oak	93%	✔
Summary	93% (949 / 1022)	✔

[ShiyuC]

Thanks for the nice fix @rammprasad ! The PR looks good to me. 💯 ⭐ 👍

[ramiromagno]

I also confirm everything is passing now.

[galachad]

Discussed with @rammprasad that we can close it as snapshot functionality works as expected.