Got it with Retell this is usually one of two things: either the agents were deleted via the API but your dashboard is caching the old list (not re-fetching), or they still show because they're tied to a phone number/campaign that wasn't unlinked first, which is also why clicking Delete fails now (Retell won't let you delete an agent still attached to something).
Quick check: are these test agents still assigned to any phone numbers in Retell, or already unassigned?