When you download an extension (OXT) file and then right-click it in the file manager, it will be associated with LibreOffice (Start Center). If you try to open the OXT file directly from the file-manager using the start center, a weird workflow will happen:
Steps to reproduce:
1) Right-click the OXT file in the file manager and choose "LibreOffice" (choose the start center)
2) When the start center opens, the extension manager will be automatically opened. Click "OK" to install the extension (also accept the extension license)
3) Close the extension manager. Then you will receive a message box saying that you need to restart LibreOffice.
4) Choose "Restart Now"
5) Here's the weird part. LibreOffice will restart and after the start center opens again, it will ask if you want to install the extension (again) as though you were opening the OXT for the first time.
The expected behavior would be to restart LibreOffice and open the start center without asking to install the extension again.
Notice that this does not happen if you install the extension via Tools - Extension Manager.
This is repro at least since LO 7.2.x (I tested in Linux and Windows) and it is still repro in the latest master (LO 7.5+).
*** This bug has been marked as a duplicate of bug 117100 ***