Bash not interpreting negative offsets in Substrring Expanison of parameters

According to the man page, $(variable:offset[:length]} should treat negative offsets as counting from the end of the value. Instead, I find any such substring expansion expands the ENTIRE variable regardless of the exact negative offset or any length provided.

A short typescript is attached. It shows two good expansions followed by three bad ones.

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: bash 5.0-6ubuntu1.1
ProcVersionSignature: Ubuntu 5.4.0-66.74-generic 5.4.86
Uname: Linux 5.4.0-66-generic x86_64
NonfreeKernelModules: wl
ApportVersion: 2.20.11-0ubuntu27.16
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: XFCE
Date: Tue Mar 23 09:09:28 2021
SourcePackage: bash
UpgradeStatus: Upgraded to focal on 2021-02-19 (32 days ago)