The private key displayed for old addresses in the Imported Addresses list is a private key for a compressed public key, even if the addresses is for an uncompressed public key. The uncompressed form of the private key should be the one that is shown.
Here is a screenshot illustrating the problem:
https://i.imgur.com/GVXUPkf.png
As you can see in the screenshot, a compressed private key is given. However actually importing that private key into other wallets will yield a different address. Putting it in bitaddress.org reveals the problem; the address displayed (and used) is for the uncompressed public key, but the private key given is a for a compressed private key.