Can confirm, authentication files don't help. Bing is seriously broken, and their support is all but helpful.
This is an excerpt from one of their replies:
> "Thank you for your patience during our investigation. After further review, it appears that your site did not meet the standards set by Bing to remain indexed the last time it was crawled. To ensure that this was not a false flag, I also escalated the issue to our Product Team and they manually reviewed your site and confirmed that it is in violation of our Webmaster Guidelines detailed here:
https://www.bing.com/webmaster/help/webmaster-guidelines-30f....
We are not able to provide specifics for these types of issues but we recommend that you review our Webmaster Guidelines, especially the section Things to Avoid, and thoroughly check your site for any deliberately or accidentally employed SEO techniques that may have adversely affected your standing in Bing and Bing-powered search results."
Before snarking, please check that link and the long lists of things - I did not find to find my website https://linmob.net to be offending their "things to avoid list".
That was a reply to my first ticket requesting re-indexation, later tickets only got what I would call "non-replies".