The missing + and - is not a UI bug. A version in Ready for Review is already inside a review submission, and its build relationship is frozen while that submission is open. The buttons come back once the version is out of it.
Two ways out, depending on what you want.
If you want to keep the submission, cancel it first. In the web UI that is "Remove from Review" on the version page; over the API it is PATCH /v1/reviewSubmissions/{id} with canceled=true — note this only works on a submission that has actually been submitted. Then attach the build and submit again.
If the version was rejected rather than waiting, just attach the new build. That alone moves the version back to PREPARE_FOR_SUBMISSION and an "Update Review" button appears, which resubmits with the new binary.
On your other question, deleting the version: I would not. The review thread hangs off the submission, and Apple exposes no DELETE for reviewSubmissions at all, so deleting the version is a bigger and less reversible move than cancelling the submission.
When you do attach the build, read the relationship back rather than trusting the response. PATCH .../relationships/build returns 204 either way; GET /v1/appStoreVersions/{id}/build is the actual check. Script that does both, MIT, no dependencies: https://github.com/vskromny/appstore-release-kit
Topic:
App Store Distribution & Marketing
SubTopic:
App Store Connect