eriktorbjorn is correct.
Using 2.6.0git4526-g1d1a12b3aca, i remove and added again zak-fm in both english and spanish (fan translation) and now in both appears the setting.
Is strange because when i added them before (it's been a while) it was in a consecutive way: copying the files from GOG, copying the folder twice, applying the spanish patch to one, and adding both to ScummVM at the same time. Maybe i'm remembering wrong, but anyway, the settings appears now in both.
But it's true, the Loom FM-Towns doesn't add the option. As seen in https://github.com/scummvm/scummvm/blob/master/engines/scumm/detection_tables.h#L181, the detection lacks the GUIO_TRIM_FMTOWNS_TO_200_PIXELS flag.
If it's intended that flag can be used in Loom FM-Towns for any reason yet, this bug can be closed. I edited the bug to reflect the issue now it's about Loom FM-Towns only.
Thanks.