The cyan loading shader would in some cases leak into baked reflection probes. This problem may be a general shader/material problem. From studying the reflection probe behaviour, it seems that the loading shader is sometimes picked for drawing in cases where allowAsyncCompilation = false and async compilation is already ongoing. According to the semantics of allowAsyncCompilation this should not happen.

This issue does not reproduce on trunk as of Sep 24th. Repro verified in BiRP and URP. HDRP was not tested.

Remark 1: The reason that the scene view must be visible in the repro is probably that this triggers the asynchronous compilation. If you bake a reflection probe without triggering an asynchronous compilation (e.g. by hiding scene and game view) then the issue does not happen.

Remark 2: If you cannot repro the issue, you can try to clear the GI cache.

Attachments:
CyanLeak.zip: A simple Unity project that contains a sphere and reflection (as described in the repro above).
cyan-leak.mov: A video showing the repro in action.