I used the Share for Review option for a document I'm working on: the file is public and pw protected.

Several people told me that the comments they add are not mapped even though they specifically use the "Place a pin" option. The comments I receive show as "Unmapped comments". I tried it myself with the same results.

I'm working on a 200+ pages document and really need this fixed.

Same in my case too, some "unmapped comments". Sometimes it works, sometimes not.
Unfortunately you can't offer this tool to my customers. and me neither...
When it works, it´s very cool. But if not, the additional work is horrible.

I am experiencing the same issue, too. Wired: Till page 69 everything works well, since page 70 all comments are unmapped. I am working on a doc with around 100 pages, many bigger pictures. Tried to fix the problem by deleting items step by step from page 70/71. no success. When I copy these items (p 70/71) in a totally new doc, everything is running well in this small document. Deleting pages 1-63 in original document (chapter ends on p 63), ALL pages run into this issue again.

Same issue here, using a Chromium-based browser (Min).

— Was previously working as expected, then ran into this issue when InD crashed (https://indesign.uservoice.com/forums/601180-adobe-indesign-bugs/suggestions/40698859-share-for-review-white-black-screen-only).
— Performed steps outlined in the linked post, but did not work for the file that was open when InDesign crashed (and assumed I had done it wrong).
— Duplicated and renamed the file (for a different reason), and I was suddenly able to create a Review link again. This is when I ran into the above issue.

same issue here using chrome browser, this renders online reviews useless.

I am experiencing the same issue.