Happened on this question, fwiw. In particular I voted to reject it, then visited to add comment and vote to close, and wanted to add a java tag, but couldn't because had to wait to see the fate of the edit.
The problem is that in the general case a single reject vote could be an outlier. If the edit was subsequently approved any other edits that occurred in the intervening period would introduce a three-way merge.
Allowing just the edits that wouldn't conflict means rejecting the ones that would. The error message "sorry your edit might conflict" is less intuitive and technically harder than blocking all edits whilst a review is in progress.
(There's also the question that if you allowed this should multiple non conflicting reviewed edits be allowed to be pending simultaneously?)
In short most edits should be fixing all the issues anyway and if you're that desperate to edit before review is over you can go through improve, untick helpful and then revert the edit manually as a binding "reject+edit" action.
In the specific instance I don't quite follow the logic of "fix the tags but reject fixing any other issues" either. An improve alone would be sufficient, with unhelpful as appropriate depending on if you think the suggestion was too minor.