Master Property Undo and Copy Expression Bug
If you replace a comp that has a master property with an expression or keys on it, it gets cleared out. That makes sense. But if you undo, expressions and keys on a mastery property don't return.
Also, the UI lets you "Copy Expression Only" for Master Properties but you can't paste onto duplicates, even of the same precomp with the exact same Master Properties.
Aside from a few minor issues, I love the world Master Properties opens up for us.

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Peter Bokor commented
I've just run into this that "copy expression only" doesn't work from one precomps master property to another precomps identical master property. Would love to see this fixed. Thanks.
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Aurelijus Čiupas commented
I've also run into cases where I wanted to "Copy Expression Only" for Master Properties but they won't automatically paste on selected layers with the same MPs. It's a very helpful feature when pasting expressions to large quantities of layers at once without having to twirl down to the exact property and with MPs it is not functioning.