I am able to equip and play with weapons that I have yet to unlock.

This only works if you have at least one package unlocked for the weapon.

Steps to reproduce:

1. Open any class loadout.
2. Press on the Primary Weapon to open the weapon selection to change weapon.
3. Find the desired locked weapon.
4. Click on the locked weapon in the weapons selection.
5. When the locked weapon is selected, click "X" on your keyboard or the "Modify" button.
6. Now that you're on the package selection for the weapon, simply double click the unlocked package and you will have the weapon equipped, and you're now able to play with it.

I've been playing the new REDSEC, and because of that, I had a lot of unlocked attachments to weapons that I had yet to unlock. I then found out that I can still "read" those notifications, but pressing on a locked weapon and pressing "Modify". While going through all the locked weapons and attachments I had unlocked, I noticed under the PSR, that I had unlocked a package for it, from the Battle Pass, so my natural reaction was to double click it to clear the notification, and that's how I found out.