Right-click on a preset, instrument or sample and select "Rename...", and a rename window pops up. In previous versions of Polyphone, the text field would be active, so I could just start typing. However, in 2.1.2, the "Ok" button is highlighted instead, so it requires a mouse click in the text field before I can actually rename the element.
After testing with different Linux distributions in VirtualBox, it would seem this bug is due to my system having Qt 5.13.2. Systems using Qt 5.12.4 and earlier do not have the bug.