Thank you for reporting a bug to Unity.
We have looked into the repro case, and concluded that this case is technically by design.
In URP, when using MSAA and alpha clipping, the new edges of the object caused by the cutout are not taken into account for antialiasing. In this case, URP will automatically enable a2c(alpha to coverage) for the opaque pass to benefit from MSAA.
The a2c will make opaque pass writing to the alpha channel, and this channel is later used as a coverage mask during the MSAA resolve. You could find more detailed explanation about the technique here: https://en.wikipedia.org/wiki/Alpha_to_coverage
This alpha-to-coverage feature is technically working as expected. However, it is indeed incompatible with the Composition layer use case, where the alpha channel is used for deciding where to blend with the comp layer. We suggest using a workarounds:
1. Make the object transparent - this should disable the A2C feature or
2. Disable MSAA - this should disable the A2C feature
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.