Azure pipelines node version discrepancy

I'm fairly new to Azure Pipelines and CI/CD in general. My pipeline needs to install node, then PNPM, the dependencies, and then run a build script.

Here's my pipeline YAML:

trigger:
  - none

pool:
  vmImage: ubuntu-latest

variables:
  pnpm_config_cache: $(Pipeline.Workspace)/.pnpm-store

steps:
  - task: UseNode@1
    inputs:
      version: "20.x"
    displayName: "Install Node.js"

  - task: Cache@2
    inputs:
      key: 'pnpm | "$(Agent.OS)" | ./src/ui/pnpm-lock.yaml'
      path: $(pnpm_config_cache)
    displayName: Cache pnpm

  - script: npm install -g pnpm
    displayName: "Install PNPM globally"

  - script: pnpm config set store-dir $(pnpm_config_cache)
    displayName: "Setup PNPM caching"

  - script: |
      cd src/ui
      pnpm install
    displayName: "Install PNPM packages"

  - script: |
      cd src/ui
      echo Node version
      node -v
      echo PNPM version
      pnpm -v
      pnpm fastbuild
    displayName: "Build UI"

The first five steps all run without a hitch. But then during the "Build UI" step, I get the following:

========================== Starting Command Output ===========================
/usr/bin/bash --noprofile --norc /home/vsts/work/_temp/8e8e15dd-3e1b-4d6f-abe1-a994bcc1669a.sh
Node version
v20.12.2
PNPM version
9.0.6

> pov-ui@1.0.0 fastbuild /home/vsts/work/1/s/src/ui
> pnpm lib && gulp build:fast --target=integration

ERROR: This version of pnpm requires at least Node.js v18.12
The current version of Node.js is v16.20.2
Visit https://r.pnpm.io/comp to see the list of past pnpm versions with respective Node.js version support.
 ELIFECYCLE  Command failed with exit code 1.

##[error]Bash exited with code '1'.
Finishing: Build UI

As you can see, I've added some debug lines to confirm that the current version of node is the one I installed earlier (v20.12.2). But as soon as the PNPM script starts to run, it seems to fall back to v16.20.2.

Can anyone help me understand why PNPM is using a node version other than the one I explicitly installed?

Note: I realise the documentation for PNPM suggests using Corepack to install PNPM, rather than the npm install -g pnpm approach I've gone with here. But whenever I used Corepack, it was giving its own inscrutable errors. All the suggested fixes for that particular issue suggested updating to a newer version of node, which didn't help me as I thought I was using the current LTS version. Now that I discover I'm not, the error's making more sense.

On ubuntu-latest agent, it already has node.js version 18.20.2 by default(link here).

Checked on my side, with your npm install -g pnpm task, it can install 9.0.6 version PNPM, and pipeline can work fine:

The current version of Node.js is v16.20.2

It could happen that you have v16.20.2 node.js defined somewhere, eg .npmrc.

It will fetch the version v16.20.2 in this case:

Please check the code if v16.20.2 node.js is defined, fix it or remove it for a check. In addition, please temporally remove the cache task to avoid code restore, you can add it back once pipeline is working.