Word can show manually added bookmarks but doesn't show them if they are 'hidden' (ie their name starts with "_". You show those bookmarks by going to File > Options > Advanced > Show document content > Show Bookmarks

I suspect the problem you encountered is because you inserted the new item by placing your cursor at the start of the line and pressing enter. Then you moved up to the new empty paragraph and added your new #18. This effectively expanded the bookmark to include both the #18 and #19 endnotes. What you SHOULD have done was to put your cursor at the end of the paragraph above and pressed enter. This would have inserted the new paragraph outside the bookmark.

If you set the option to show your bookmarks and add a manual bookmark you will see grey [square] brackets appear which show where that bookmark is. Now put your cursor at the position where that bookmark starts and see what happens when you insert content there (the new content is inside the bookmark). This is the opposite of what happens when you add content where the bookmark ends. This same behaviour is what happens with hidden bookmarks (but you just can't see them).

When you have a bookmark which includes multiple paragraphs, a cross-ref to that bookmark which asks for the paragraph number returns the number of the first paragraph only.

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Andrew Lockton
Chrysalis Design, Melbourne Australia