I have a similar problem on mac mini 2018 Mojave. I sent an email to support team and they said that:
"In order to make League run as it should on your system, your best bet would be to use Boot Camp. Just download the Windows 10 version and you should be good to go.
Also, as a word of advice, you can get a free copy of Windows on Microsoft's website, so even though you don't have a key, you can just skip that step and install it on your machine. This will allow you to play League of Legends while also taking advantage of all the power of your MAC. I know, I know, it means playing the game on a completely different OS. While this is not the ideal solution, it should allow you to enjoy the game without any major hiccups, at least until some changes are going to happen with Apple's latest OS. ^_^"
I'm very upset, RIOT :(