For chemical cells tooltip shows a word "Precise" instead of chemical name

That's just how partitioning works in AE2 altogether. "Precise" here means that the cell only accepts the exact contents it's partitioned to, as opposed to "Fuzzy" (with a Fuzzy Card) where it will accept any item of that kind but with any extra/different data such as damage and other components.

For radioactive materials (from MEGA Cells mod) there is normal material name. But yes, may be it's a bug of AE2.

Again, it's not a "bug", it's just unclear wording.

Also, MEGA's radioactive cell uses a bespoke inventory with custom tooltip logic to begin with. It's entirely separate from other regular storage cells for items, fluids and chemicals.

I do not insist on 'bug' classification. It would be good, if app-mekanistics implements for chems custom tooltips like MEGA Cells and AppliedFlux. Chems cannot be "precise/fuzzy". Fluids too, but fluids is AE2's part.