Helm null handling has regressed. It's noticeable in Test 2.

Each test can be run by Helm 3.17.0, and Helm 4.1.1. Compare the outputs!

alias helm-317='podman run --rm -it -v $PWD:/app -w /app "alpine/helm:3.17.0"'
alias helm-411='podman run --rm -it -v $PWD:/app -w /app "alpine/helm:4.1.1"'

• The child chart defines a config section (empty mapping) in values.yaml.
• The parent chart defines a config value in values.yaml
• The custom next-values.yaml overrides the parent config key with null

(
  cd nested-chart-values-override
  helm-317 template . --values=./next-values.yaml 
  helm-411 template . --values=./next-values.yaml 
)

• The child chart defines a config section (empty mapping) in values.yaml.
• The parent chart defines an arbitrary key in values.yaml
• The custom next-values-1.yaml and next-values-2.yaml define a different key with a non-null, and null value, respectively.

(
  cd nested-chart-siblings-override
  helm-317 template . --values=./next-values-1.yaml --values=./next-values-2.yaml 
  helm-411 template . --values=./next-values-1.yaml --values=./next-values-2.yaml 
)

• The chart defines a config section with a default value in values.yaml.
• The custom next-values.yaml overrides the chart config key with null

(
  cd single-chart-values-override
  helm-317 template . --values=./next-values.yaml 
  helm-411 template . --values=./next-values.yaml
)

• The chart defines a config section (empty mapping) in values.yaml.
• The custom next-values-1.yaml and next-values-2.yaml define a different key with a non-null, and null value, respectively.

(
  cd single-chart-siblings-override
  helm-317 template . --values=./next-values-1.yaml --values=./next-values-2.yaml 
  helm-411 template . --values=./next-values-1.yaml --values=./next-values-2.yaml 
)