BCP338 when referencing variables from functions

[carbage]

Bicep version
Bicep CLI version 0.29.47

Describe the bug
Chaining variables causes BCP338 - removing subscriptionId from the diagnosticSettings object fixes the error.
Full error for this example:
Failed to evaluate parameter "keyVaults": Unhandled exception during evaluating template language function 'variables' is not handled.

To Reproduce
common.bicep

@export()
func diagnosticSettings(env string) object => {
  eventHub: {
    namespace: {
      subscriptionId: subscriptions.default[env]
      resourceGroupName: 'diag-rg'
      name: 'diag-ehn'
    }
    name: 'metrics'
  }
  metrics: [
    {
      category: 'AllMetrics'
    }
  ]
}

@export()
var subscriptions = {
  default: {
    'dev': 'xxx'
    'uat': 'xxx'
    'prd': 'xxx'
  }
}

key-vault.bicepparam

using 'br/modules:key-vault:latest'

import { diagnosticSettings } from './common.bicep'

var location = 'uksouth'
var locationSlug = substring(location, 0, 3)

var env = readEnvironmentVariable('ENVIRONMENT', 'dev')
var diagSettings = diagnosticSettings(env)

param keyVaults = {
  'key-vault': {
    resourceGroupName: 'resource-group'
    diagnosticSettings: diagSettings
  }
}

Additional context
I've noticed a similar issue, for example, when using loadJsonContent('./some/json.json') in place of imports though have not reproduced for this specific issue

[carbage]

FWIW, this can be resolved in this scenario by replacing subscriptions with a function returning object

@export()
function subscriptions() object => {
  default: {
    'dev': 'xxx'
    'uat': 'xxx'
    'prd': 'xxx'
  }
}

The inverse, referencing functions from exported variables also works, though the case in question would be fairly beneficial.