Thought I'd post this as a warning so people won't suffer what I have.
So, this story starts yesterday, which I had a world glitch which made the game forget my save existed, despite it being in the correct folder and everything. Found a dodgy, but ulimately successful fix, on this very forum no less. Everything seemed to be fine, and I was very happy.
Today, I was playing this world for another few hours, working on getting a new project up and running.The world started to get laggy, a quick check showed it was a RAM thing, so I just restarted the world as a temp fix. However, when I got back in, all those hours of progress had been completely rolled back. Nothing in my inventory was wrong, but every interaction I'd had with the world, block placing, item moving, chest sorting, it had all been undone.
I ran a few tests and quickly realized the unfortunate problem. Turns out that dodgy fix meant the world will no longer save progress. Or some other bug occured that led to it. I'm not sure of the cause, although I am sure there's probably some very technical thing I could do to fix this, but I wasn't nearly invested in this particular world enough to care. I just deleted it, and I'll start over again soon after I care enough to play again.
But I'm posting this here for two reasons: firstly, to see if anyone does have one of those technical fixes for people who are further along than I was, and secondly, as a firm reminder of how important world backups are with mods. That may not need stating at all, and may be super obvious to everyone. But I'll say it anyways, cause its still a good idea.
May the rest of you never suffer this unfortunate and strange bug, and I hope you have a good time tekkiting.