"Last seen" on WhatsApp for same contacts on different phones is not shown
There is a setting, which allows you to control who can see your "read receipt" or your "last seen" (or a few other settings).

You find this settings under:
WhatsApp > Menu Button > Settings > Account > Privacy.

Following settings are possible: Everyone, My Contacts, Private.

To summarize your described behavior: Your observed person must have one of your numbers in his contact list, and has selected "My Contacts" for this settings (regarding "last seen"). So you are able to see the "last seen" from the number, they have in his contact list. But not from your additional number

For additional information you can visit https://www.whatsapp.com/faq/en/android/23225461