XCOM 2: 15 Things You Didn't Know You Could Do
8. You Can Unlock Hidden Abilities By Building The Advanced War Centre
Its actually possible to unlock a soldiers hidden abilities when first constructing the Advanced War Centre, but most players dont know this because the centre is usually constructed fairly early on, and most hidden abilities are higher levelled. So, if you actually tear down your AWC and construct an entirely new one, theres a chance you can unlock these abilities. It's also possible to unlock abilities from different classes, meaning that you can potentially unlock the Chain Shot ability for a Specialist, a move typically only available for Grenadiers.
Its probably the easiest way to unlock these abilities, and its strange that the game denies them to you in such a weird way. Likely, itll be patched at some point, but until then, you've got to tear them down to build 'em up.