the failing tests are because the testsuite currently relies on unmapped public properties to track the usage of postLoad listeners.
For doctrine/common proxies, public properties were unsupported and were not triggering initialization (public methods were the trigger, and using a public property was allowing to skip using any method), which was a known limitation of the proxy system (meant to be solved by the new system, among other things)
For the initial implementation of the lazy-ghost proxies, mapped properties were the trigger, making those tests work as well (as those are unmapped properties). But as this PR is precisely about changing the trigger to be all properties, tests will need a different way to track usage.

Would it make sense to do this only when there are postload listeners?
Alternatively, it means we should "skip" the properties used in tests, right?

I wanted to refactor those tests to store the initialization tracking externally, but I haven't done it yet (I kinda forgot this PR)

/cc @priyadi

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Is there perhaps an update for this issue?

I haven't had the time to go back at it to update the tests. But I saw that #11853 had similar needs to update the tests, so I might be able to borrow from it.