Inconsistent ability to modify Outline Level of custom Headings

I'm using Word for Mac 15.31 in MacOS Sierra 10.12.3, but this issue appears to be cross-platform, since a Word-for-PC user has experienced it as well (see end of this post):

Here's what should be the procedure for modifying the Outline Level of a custom Heading:

1. In the Styles Pane, click on the down-triangle next to the Heading whose Outline Level you want to adjust.
2. Then select:  Modify Style –>  Format –> Paragraph –> Indents and Spacing –> Outline Level.

However, usually the Outline Levels are grayed-out and thus not changeable (for both built-in Headings and custom Headings).  The only procedure I've uncovered that consistently allows you to change the Outline Level for a custom Heading (this doesn't work for built-in Headings) is the following workaround.  Suppose you want to change the Outline Level of "Custom Heading 1":

1. Highlight an existing Heading, whose style is "Custom Heading 1",  in the document.
2. Go into the document's Paragraph dialog (as opposed to the Heading's Paragaph dialog), and change the Heading's Outline Level there.  
3. Go back to the Styles Pane, click on the reverse triangle for Custom Heading 1, and then select  "Update to Match Selection".  

This roundabout method is obviously unnecessarily complicated and laborious — there's no good reason you shouldn't be able to do this directly, in the Styles Pane, with Modify Style –>  Format –> Paragraph –> Indents and Spacing –> Outline Level.  After all, why does Word offer "Outline Level" as an option under Modify Style, only to leave that option (usually) disabled?  

[Note that all my custom Headings were created based on styles of built-in Headings.]

Further, as I mentioned above, when it comes to built-in Headings, even this procedure doesn't work.

But here's the additional puzzle:  One's inability to directly modify the Outline Level of custom Headings (using Modify Style) is not consistent.  For instance, suppose you click some body text, and then change that text to a custom Heading style.  Then, usually, if you try  Modify Style –>  Format –> Paragraph –> Indents and Spacing –> Outline Level, you'll find that this custom Heading's Outline Level is no longer grayed out!  [I.e., you just have to first change some text to that Heading, and then you can internally modify the Heading's Outline Level; the problem is if the text already is in that Heading style, in which case Outline Level will typically be grayed-out.] As a further oddity, if that custom Heading was based on a built-in Heading, if you go to the corresponding built-in Heading, you'll usually see its Outline Level is no longer grayed out either!  But if you do some other stuff in the document, and then return to check either the custom Heading or the built-in Heading, you will see that their Outline Level pull-downs have reverted to a grayed-out status.

An inconsistent ability to modify the Outline Levels of Headings has also been observed by Word MVP Doug Robbins, who is using Word for PC (indicating this issue is cross-platform).  He found that, for built-in Headings, sometimes Outline Level is grayed out, and sometimes it's not. Here's an excerpt from one of Doug Robbin's posts at https://answers.microsoft.com/en-us/msoffice/forum/msoffice\_word-mso\_mac/three-issues-with-navigation-pane/9933f8eb-d25b-4c42-a743-623815949d60 :

For the Built-in Heading # styles, it is not possible to modify the level. (checking again, I find that it is possible.  I do not know why the Outline level: drop down was disabled when I took the screen shot.)