I can repro the following problem 100%. Only way to recover is to import an exported copy of the RL instance, or restore a backup.
Room Light instance with time periods set.
Go into the time periods edit page, then click on a time period
Close browser tab
Return to the app instance in a new tab : rule edition is irretrievably broken (but it still works! lights are still automated)
I am aware of warnings not to navigate away from RL or RM pages in this manner; however the few times this has happened to me was on the mobile app, where my fat fingers or foggy brain caused me to hit "Done" at the top of the screen inadvertently.
I have copies of the original and broken JSON exports, if that can be useful.