To reproduce:
1) Create a new project
2) Import an .so plugin (one is attached)
3) Select it in the project view
4) Change the selection to make the plugin either set or unset for Linux x86_64 platform
5) Click "Apply"

Result: Nothing happens (or in case of older versions, it reverts on clicking Apply). If you try to close Inspector and popup jumps out asking to apply the changes, it still does nothing

Reproduced in 2019.2.0a3, 2019.2.0f1, 2019.3.0a1, 2019.3.0a12
Not reproduced in 2019.2.0a2, 2019.1.13f1, 2018.4.5f1, 2017.4.30f1