I have just tested the CH4 filter with a UV-IR cut filter. I can see the same as without the UV-IR cut filter, as expected. So there is a real problem of visible light leakage whith the CH4 filter. But I don't know if it is a filter design problem or there is a batch problem.
This leakage could affect to the image. If the problem is the batch of my filter, a new one will fix the issue. If the problem is the design, then, asuming the filter behaves as expected in te CH4 band, then a IR850 filter on to the CH4 one could be used to get rid the spectrum below 850 nm. But I do not think this is a good solution for anybody who buys a genuine CH4 filter and relies in the manufacturer.