Hey @Jonathan
I have brought this up with the Team!
How it is currently, is just as you described. There is the initial warning prompt and then if you leave and come back, there is no follow up warning.
Initially I believe there was numerous prompts, but this was removed as some players felt like it may of been a little spammy.
Also passed on the feedback that a prompt in regards to having Event medals equipped while in different game modes, would be beneficial for the players.
I have although moved this to feedback, as currently it is intended to prompt the first time you enter World Events
Jeto - Support Team Human