Timer trigger not working with user managed identity and without `WEBSITE_USE_PLACEHOLDER_DOTNETISOLATED` appsetting

#### Description
Timer trigger execution is not working with user managed identity enabled and `WEBSITE_USE_PLACEHOLDER_DOTNETISOLATED` appsetting not there. I saw some correlation with high CPU consumption potentially when the appsetting was not enabled in a CRI, but I'm still creating this issue to track more instances of this occurring.

#### Repro steps

Steps to repro

1. Create a new .NET Isolated Azure Function App.
2. Set it up with a User Assigned Identity.
3. Assign this User Assigned Managed Identity to Storage Blob Data Owner.
4. Enable Identity based Host Connections .
5. Remove WEBSITE_USE_PLACEHOLDER_DOTNETISOLATED from the App Settings.
6. Deploy a Timer Trigger
7. It will fail to run.
8. Add back in WEBSITE_USE_PLACEHOLDER_DOTNETISOLATED=1 and it will work.

#### Expected behavior

.NET isolated function app should work

#### Known workarounds

Remove the appsetting

#### Related information

Provide any related information

* Programming language used
* Links to source
* Bindings used

Start by reproducing the failure in a new .NET isolated Azure Function App with a user-assigned identity, identity-based host connections, Blob Data Owner access, and a Timer Trigger. Compare behavior with WEBSITE_USE_PLACEHOLDER_DOTNETISOLATED absent and set to 1, and investigate the reported high-CPU correlation. Done means the timer runs with the setting absent.

Written by the indexing model from the issue text.