I wanted to get to the bottom of this so I created a Word document with bookmarks. I found that Word would not let me add any bookmark with numbers in it so I am curious as to how you managed that?
The next thing I found was that on linking a document to a project, unless the document has a TOC the bookmarks don't get imported. Is that your experience?
Then I linked and found that the bookmarks in Word had been renamed to the text of the heading rather than what I had entered.

I then went back to the document and changed the headings so that they had forty, thirty and twenty characters.
On updating the document I then got this in RoboHelp. The numbers are simply because each heading have the text I used. As you say, it gets clipped on linking. The logic of my headings was that "bookmarked" and "_ookmarked" both contain ten characters so I hoped to glean some information from that.

Finally I went back to the document and removed the underscores replacing them with B.

That confirmed beyond a shadow of doubt the limit is 25, albeit RoboHelp adds a number if a duplicate gets created and a further clipping occurs with underscores.
There is no setting I can find that allows you to change that number and so far I haven't found any way of changing how this works, other than the obvious limiting the headings in Word to 25 characters.
Use the menu (bottom right) to mark the Best Answer or Highlight particularly useful replies. Found the answer elsewhere? Share it here.