It's not necessarily the server. It could be an issue between the players and the server due to routing when a critical network node is overloaded or down. (When all players disconnect from a match, the game will end.) This is not your fault. This is not Riot's fault. Riot's system doesn't know this is the issue and punishes you for it automatically. Being that loss prevention was not enabled, I will go on the assumption that no technical issues were identified with the server by Riot.
There isn't a good approach to the issue you experienced that will eliminate the possibility of people being punished for issues beyond their control, while still consistently punishing players who intentionally leave games. and statistically (ignoring exceptional cases like national disasters), this current system will punish players intentionally leaving far more often than it will punish players for issues beyond their control.
Source: Professional Software Engineer
I would contact support and calmly explain the circumstances to see what they can do about removing your low priority status. Riot understands that there are networking issues that are beyond your or their control.
Best of luck,
Illaominati