Hi Simone,
I suspect that the problem described is due to the DSP (effect block) assignment. With the SX900 only three (3) DSPs can be used for Styles (Insertion effects 7 & 8 as well as the Variation effect if this is configured as an Insertion effect).
With the SX900 preset style "CrazyPop" you mentioned, one of the two Insertion effect blocks that can be used for the Style is assigned to MIDI channel 12 (style part "Chord1", effect type "Multi FX Crunch Wah") and the second Insertion effect block is assigned to MIDI channel 9 assigned (style part "Rhythm1", effect type "Ambience Celeste"). Incidentally, the Variation effect is configured here as a "System" effect (effect type "Tempo Cross 1"), so it is not available here as an additional Insertion effect for styles.
When editing a style, the DSP assignment may change unintentionally. Therefore, before saving the changed style, check whether the desired DSPs (effect blocks) are assigned to the relevant style channels or assign the DSPs (with the desired effect types) to the relevant style channels manually.
In the case of the "CrazyPop" style mentioned, I recommend checking the DSP assignment first with the unchanged preset style and then checking the modified saved style (in "Mixer > Style page > Effect tab > Assign Part Setting".
Hope this helps!
Best regards,
Chris