The problem was first reported here:
https://fogbugz.unity3d.com/f/cases/1171325/
However, instead of fixing the UI element, the specific instance pointed out in the bug report was fixed. This UI element shows up in multiple packages and locations in the engine and would be next to impossible to track down and fix every instance; on top of this, it's possible that this is broken for custom editor UI for asset store assets as well. The actual UI element needs to be fixed. This problem exists in both 2019.3.0b5 (13eef430e0c9) and Version 2020.1.0a7 (d3d2f5b1d33e)

Examples of places I've found the bug:
1. Terrain Tools package: Create a terrain, click the terrain, click the "paint terrain" tab in the inspector, choose any tool from the menu. Attempt to click any of the arrows next to "Brush Strength" or "Brush Size". Clicking on the arrows won't work, you will have to click slightly to the right of the arrows. This effects almost every drop down arrow like this in the package. Since these elements worked on this package in 2019.2 it is probable that this bug will also effect custom UI from assets on the asset store as well.
2. With or without terrain tools package, with Universal RP 7.1.2 release: Set your graphics pipeline to a universal pipeline asset first. Create a terrain, click the terrain, click "paint terrain", choose the "paint texture" tool, create a new terrain layer, open the layer properties and scroll down to either "Channel Default Values" or "Channel Remapping" (depending on whether there's a mask map). Attempt to click the arrow to expand this section; just like previous examples, you will have to click to the right of the arrow in order to expand the section.