The blogging platform became unavailable on the technology giant's app store for both iPhone and iPad over the weekend, with no official statement from either Tumblr, its parent company Oath, or Apple.
But now, on the site's official help page, it has been confirmed that the reason for the shut-down was because of disturbing child pornography that was being hosted on the site.
Tumblr claims they "monitor content uploaded to the platform" using a database of known child sexual abuse material, but that the images that appeared on its site had not yet been included in the database.
Although the company removed the offending images immediately and deactivated accounts that had shared it, the app is no longer available on the app store, and Tumblr are "working to resolve an issue with the iOS app".
In a statement on their help page, Tumblr said: "We're committed to helping build a safe online environment for all users, and we have a zero tolerance policy when it comes to media featuring child sexual exploitation and abuse. As this is an industry-wide problem, we work collaboratively with our industry peers and partners like NCMEC to actively monitor content uploaded to the platform.
"Every image uploaded to Tumblr is scanned against an industry database of known child sexual abuse material, and images that are detected never reach the platform. A routine audit discovered content on our platform that had not yet been included in the industry database.
"We immediately removed this content. Content safeguards are a challenging aspect of operating scaled platforms. We're continuously assessing further steps we can take to improve and there is no higher priority for our team."
As of the time of writing, it is unknown whether Apple removed the app as a safeguard, or if Tumblr voluntarily took it down.