Configuration: Ubuntu 18.04 LTS, GNOME Tweaks 3.28.1 (I believe it's pre-installed in 18.04), Middle Click Paste: OFF
Problem: It's not disabling middle click paste, although it worked once (until after the first restart with that option set to OFF).
Question: What is the actual shell command that disables the MCP feature?
Reasoning behind the question: I figured that GNOME Tweaks is executing that command by toggling that ON/OFF option and that it may use the wrong command now. This might be due to a dist-upgrade that changed the command.