Updating cached assetbundles no longer working on iOS (Unity 2018.1)

An assetbundle manager inspired piece of code we've been using for over a year has recently started malfunctioning without having made any changes to it.

We generate bundles, download a manifest, and then download bundles if they weren't cached yet. This has always worked without any problems, but starting recently this no longer works on iOS. It just doesn't detect that a bundle has changed, its as if Caching.IsVersionCached just always returns true. Then, when using the Caching.ClearAllCachedVersions functionality to remove the bundle, after redownloading the (new, changed) bundle, it will still have the old one. As if it's stuck in cache somewhere. I can reproduce this across different apps which use the same code on 2018.1, but the exact same code works just fine on android..