You'll have to open a Support Request to get the capability restored.
In the future, I recommend against messing with the built-in roles. Instead, clone the role and modify the clone then assign the cloned role to the appropriate users/groups. It's a little more work at first, but avoids nasty surprises like that, plus it gives you something to fall back to.
I opened a support ticket. Splunk manually re-enabled the capability. However, they have not yet been able to determine why unchecking the edit_sourcetypes capability for the Power User role removed that capability from all roles and removed the ability to re-add it.
I will update this ticket with the fix for this bug when I get the response from Splunk.