OAuth timeout when using custom OAuth client but not default client

[chen4119]

About You:
Name: Wan Chun Chen
Company: JP Morgan

We are developing a custom data connector that uses OAuth2 auth. If we configure the OAuth client as default in Tableau server (via settings -> OAuth Clients Registry). It works as expected. However if we configure a client as custom, we're seeing this error in the logs

com.tableausoftware.api.webclient.rest.WebClientRestService - Failed to add OAuth token
com.tableausoftware.domain.exceptions.InternalServerErrorException: com.tableau.microservices.status.service.ServiceInternalErrorException: connect timed out (errorCode = 0)

Looks like it's timing out when calling POST /token endpoint. Question is why did it work when the OAuth client is default? Both are hitting the same host. Is Tableau server handling custom OAuth client any differently than default OAuth client?

Our setup do require a proxy to hit the OAuth identity provider so the time out is likely a proxy issue but why can the default OAuth client properly use the proxy while custom OAuth client cannot?

We are running Tableau server 2023.3

[lukewrites]

Internal tracking: W-14870470