Still learning about film simulation bracketing myself (and using the X-e4 in general), but assuming you only changed IQ values for the custom setting can you not then use the Q button to scroll to your plain vanila profile prior to enabling bracketing?
X-E4, Custom settings and film simulation bracketing
in Fuji X-E4 / Fuji X-E3 / Fuji X-E1 / Fuji X-E2 / Fuji X-E2s
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Still learning about film simulation bracketing myself (and using the X-e4 in general), but assuming you only changed IQ values for the custom setting can you not then use the Q button to scroll to your plain vanila profile prior to enabling bracketing?