This looks to me like it's working as intended. 

Not all permissions result in specific warning text within the Permissions UI, as noted in https://developer.chrome.com/extensions/permission_warnings where it says "Some of these declarations result in a warning when a user installs your extension." and it specifically enumerates:

"The following permissions don't result in a warning:

"activeTab"
"browsingData"
"clipboardWrite"
"contextMenus"
"cookies"
"experimental"
"idle"
"storage"
"unlimitedStorage"
"webRequest"
"webRequestBlocking"  "

Yes, this is WAI.  We deliberately omit or collapse certain permissions.

There are generally three categories of omitted/collapsed permissions:

Nuisance - not worth warning the user about.  For instance, storage is viewed as relatively harmless, and we don't prompt the user (just as we wouldn't prompt the user that a website wants to use window.localStorage).

Collapsed - We don't warn the user about the permission because another permission has equivalent and greater permission.  For instance, we don't warn the user about the cookies permission when the extension has the <all_urls> permission, because if the extension can read and modify all the data on the websites you visit, that includes reading and modifying cookies (a subset of data).  Without the <all_urls> permission, we warn about cookies, but with it, we omit the warning so that we don't display n different warnings with similar content, and instead make it easier for the user to focus on the important points.

Supplementary - Some permissions cannot be used without another permission.  For instance, webRequest, by itself, does not let the extension do anything; it can only be used on the sites to which the extension has access.  Then, once the extension has access to those sites (through host permissions), the webRequest capability is captured by the host permission warning ("can read and modify...").

All of these permissions seem to fall into one of these categories.