Mystery Key Activation Issue - "Steam failed to install (connection time out)."

GMG has a sale on right now that gives you an additional Steam key for each purchase you make in the store. Unfortunately, they don't tell you what game each key is for, so the only way I can get information from it is through activation. As such, I've gone through my list and attempted to activate each of the mystery keys.

Some simply activate, which is expected. I didn't own them so they get added to my account.

Some give me the standard "Product Already Owned" message (meaning the game is already in my library), and I can then tell what game is associated with that key by the install prompt that follows (e.g. "Product Already Owned" > Install Frozen Synapse Prime lets me know the key is for FSP).

However, some give me the "Product Already Owned" message and give a long delay when I advance. An error message then pops up saying "Steam failed to install (connection time out)."

This doesn't appear to be based on my connection, as it consistently happens for the same keys, and I can activate and install others easily alongside these, which continue to fail even after repeat tries.

Anyone have any insight into this?