See, when you create a blog on the website, you can mark it as "Explicit," and that's apparently what some of the affected uses who post LGBTQ+ content did. While not all their posts are NSFW, Tumblr classifies anything from an Explicit blog as sensitive and hides the whole thing when in Safe Mode. To solve that issue, Tumblr now classifies posts individually "as they should be."
In addition, anything an Explicit Tumblr reblogs used to get marked as sensitive, even if they originally came from non-Explicit accounts. As a result, a lot more squeaky clean posts got caught in the filter. Going forward, the website will no longer mark a post as sensitive just because of a reblog if it was from a safe account in the first place.
Safe Mode also tends to hide photosets, which are always marked as sensitive until the poster submits a request to have a human staff member review all the pictures in it. Unfortunately, we'll have to wait a while for a fix to that one: Tumblr is working on a way for its algorithm to be able to review multiple pictures as a set.
While Tumblr's solution isn't complete, it has at least acted much faster than YouTube, which took a month to roll out a fix. YouTube went so far as to tweak its policies to make sure the mishap doesn't happen again, though -- we hope Tumblr is as committed as the video platform seems to be.
*Full disclosure: Tumblr is Engadget's sister brand through Verizon's Yahoo purchase.