I was running into the same issue and did some more digging myself. Appears to be an issue with when this deprecation happens.

https://github.com/php/php-src/issues/17422

As an example I've put two 8.4 deprecations together and set error reporting to 0. The implicit null deprecation still outputs an error but the unserialize deprecation does not. Example code.

For us this means php 8.4.5 (current version) is not yet production ready as we don't have control of every package to fix what is only a deprecation and extra output to stdout ruins json output.

Recommendation: stay with php 8.3 until this is fixed.