Helps to play staccato - short, clipped notes - to give the filter a chance to close again. Sounds like you're slamming it open and keeping it open with stream legato or fast notes. You can also mess with other controls, like the decay and sensitivity to try and dial it in more. I personally never got along the the MXR filter, not my cup of tea. Every filter is pretty different. Might try a proton or the source audio BEF.
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