download neon user edition ISO create a bootable pendrive using the downloaded ISO start your computer using your bootable pendrive my summary describes what happens
I experienced such behavior. Once booting iso image after couple seconds I could see NEON installer opened then disappear and Plasma continue load.
just tested neon user edition lts (plasma 5.12) and noticed the same thing.
cannot reproduce with neon-user-20191128-1117.iso