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When filters have non-parallel surfaces and/or some of their layers (and some nebula filters can have up to 75 layers (!)) are not parallel, a lateral shift can occur with wavelength, resulting in the lateral offset of the red passband versus the blue-green/blue.
This could be due to a "sandwich construction" (two pieces of glass with layers between) which was common on early Lumicon filters and several others.
And it can simple be due to a filter that is tipped in the housing (call it 'wedge"). You can try loosening the retaining ring and tapping the housing to seat the glass, then lightly re-tighten the retaining ring.
Or, replace the filter with one that has no visible red transmission (example: Orion Ultrablock).
One way to test the tilted glass hypothesis is to look at a bright star with the filter held in front of the eyepiece. Try tipping the filter to see if the colors can be "stacked". If it requires quite a bit of tilt to do so, it is likely the two sides of the filter glass are not parallel, and the filter should be replaced.
Normally, your eye won't focus the red and blue green at the same time, so some stars will appear as a blue-green point with a bit of red fuzz around them. But the colors should be coincident.
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