Thank you for reporting a bug to Unity.
In this case, the Canvas is set to Screen Space - Camera with a low Plane Distance (0.05), which causes it to be clipped at certain aspect ratios. Increasing the Canvas Plane Distance to 0.07 resolves the issue and prevent excessive bloom behavior.
We will close this case as 'As Designed.' If you have feedback on how the feature could better meet your needs, please let us know — we value your input and consider it in future improvements.