This happened to a team I was on with Google Safe Browsing. The company was a competitor to Google in certain industries. An internal domain was flagged after a security researcher set up an intentionally vulnerable service and visited it in Chrome. The particular domain was shared with some other internal services which caused an internal outage. We registered in the Google Search Console, then leveraged some industry contacts and later our legal team (which had a friendly backchannel with Google's legal team) to escalate within Google. I believe our domains were given special treatment after that.
At the time, there was no "Safety Report" to indicate why Bing thought it was dangerous. The report page linked to https://www.bing.com/toolbox/bing-site-safety?url=https%3a%2... and it said "That web page doesn't exist"
To fix it, we had to register with "Bing Webmaster Tools" (https://www.bing.com/webmasters/about) and raise a support ticket.
Within a few days, the issue "resolved itself". It's possible that raising a ticket forced some automatic refresh of the indexed data for the domain.