Persistent `No Fill` Error After Unity Ads SDK Integration with Custom Mediation Adapter

I'm encountering a frustrating technical hurdle with Unity Ads after a recent SDK integration, and I'm hoping someone here might have faced something similar.

• Context: We recently updated our application to Unity Ads SDK 4.x and integrated it with a custom mediation layer specifically for interstitial ads. The goal was to optimize our ad stack and improve overall fill rate.
• Core Problem: We're consistently receiving a "No Fill" error, particularly noticeable for certain geographic regions and device types, despite the ad unit being active and having a supposedly healthy eCPM floor. Basic troubleshooting (ad unit IDs, network connectivity, test mode) confirms it's not a simple setup error. This issue severely impacts our monetization in key markets.
• What I've Tried:
  • Verified all ad unit IDs and placement settings within the Unity Dashboard multiple times.
  • Confirmed robust network connectivity and device locale settings during testing.
  • Tested extensively with Unity Ads test mode, which consistently works fine, strongly suggesting a live production issue rather than an integration fault.
  • Thoroughly checked for any conflicting SDKs or potential mediation adapter issues, but nothing obvious has surfaced.
  • Monitored eCPM floors and bidding strategies, adjusting them for flexibility, but the "No Fill" error persists even with generous settings.
• Technical Data (Simulated): Here's a simulated console log snippet illustrating the `UnityAds.Show(...)` call leading to the `UnityAdsShowError.NO_FILL` callback, along with relevant initialization logs.

// SDK Initialization Logs (Simulated)
UnityAds: Initializing Unity Ads SDK v4.8.0...
UnityAds: SDK initialized successfully for Game ID: 1234567, Test Mode: false.
UnityAds: Ad unit 'Interstitial_Android' registered.

// Ad Load Attempt (Simulated)
UnityAds: Attempting to load ad for placement 'Interstitial_Android'.

// Ad Show Attempt and Error Callback (Simulated)
UnityAds: Showing ad for placement 'Interstitial_Android'.
UnityAds: OnUnityAdsShowFailure Callback:
  PlacementId: Interstitial_Android
  Error: NO_FILL
  Message: No ads available for this placement in the current context. Check fill rate, geo-targeting, and eCPM floors.

• Specific Questions:
  • Are there known intricacies or common pitfalls with Unity Ads SDK 4.x's fill rate behavior when integrated with custom mediation, especially concerning `geo-targeting` or `device segmentation` that might not be immediately apparent?
  • Could this be an obscure configuration in the Unity Dashboard I'm overlooking that specifically affects live `eCPM` bidding dynamics for non-tier-1 regions, leading to a perceived lack of inventory?
  • Beyond standard `SDK initialization` logs, are there any advanced debugging steps or specific Unity Ads API calls that could expose the root cause of these "No Fill" errors in a live environment?
• Really need some expert insight here. Help a brother out please...