Blocks on contentareas can be duplicated when moved around

Description

Steps to reproduce
1. Set up an Alloy site using the Visual Studio plugin (this makes a site in 9.2).
2. Log in to edit view and test dragging and dropping blocks to change their order.
It doesn't matter how you drag and drop them, the ordering changes.
3. Update to latest version of Episerver using NuGet (9.4 at the time)
4. Upgrade the database as instructed.
5. Again, in edit view, drag and drop blocks changing their order. Now, the blocks are duplicated, unless you hit the dark blue "underline", in which case the block is moved.

Expected: Shouldn't become duplicated.
Actual: Duplicated.

Note: Happens on cms.ui 9.3.5 as well.