A certain level of blurring is expected and unavoidable in general unfortunately. Note that the amount of blurring depends on voxel size, and thus should be less noticeable at higher resolutions.
Half frame offsets are in a sense the most difficult to interpolate (the minimum distance between input frames is maximized right in the middle). So your settings are giving the retimer a really difficult job here–it constantly switches between the “nicest” (integer frame) and “worst” (half frame) volumes. If you change your start frame to 1.25 instead, then you'll get a fairly consistent level of blurring throughout (since it'll compute frames 1.25, 1.75, 2.25, etc.), and the flickering should be eliminated.
Another point to keep in mind is that the viewport may occasionally lie to you about your volume's look (as in, you'll get quite a different result when you go out to render the frame). You can increase texture size limits and use HDR to avoid this.
I hope this gives you a better picture on caveats of volume retiming. Let us know if you run into any other issues!