Have been having the exact same issue but mine is a spike from 79 ping steady to anywhere from 83-102, never any higher. I pinged my ip, checked packet loss via multiple other methods, and there is absolutely none.
I have never had this issue with league of legends in over 7 years of playing, from different locations. I can currently play Fortnite, Overwatch, Battlerite, and any single other game I have tried with 100% steady ping.
I forwarded all ports for league of legends, and even attempted setting my router firewall to dmz mode via router settings. I have tried every single league of legends connection troubleshooting that riot suggests. I reset my WINDOWS to factory settings and reinstalled the game. Literally nothing fixes it. Riot employees will respond to these plethoras of posts that have been popping up (and getting deleted by mods on Reddit, by the way. Two of my own in the past day) and say the issue is on our end.
It is not. It is 100% them forcing us to connect to voice chat server that is separate from or not exactly the same as the typical league server. This can be proven when you disable voice chat in the settings in the client, then go into game with friends in a lobby. If you press M whilst in game, you can still connect to voice chat
EVEN though you attempted to disable it via client settings. This means you are forcibly being connected to something, although I'm not quite sure what.
The fact that these lag issues happen even in custom games furthers the point that this arguably useless addition of voice chat (discord is there for people partying up as well as many other options. This is not and will not be enabled for solo queue so why?) is causing network issues. And riot just has no response for us anywhere. For the first time in 7 years I guess I'll just quit league, because this is unplayable and seems malicious.
EDIT: some people keep saying this is a spectrum issue and they are wrong. I have at&t, so this isn't an isp issue just throwing this out there.
It is honestly insane that there have been over 20 posts with similar stories, as well as a plethora of comments, and riot is simply REFUSING to answer them. I see red responses here all the time that happen far swifter than this for far less complicated answers. Shame on you frankly riot. Shame.