Why do I have to reset every single source of light, every single time I open a map with the updated dynamic lighting?

Before with the "legacy", it'd save, as easy as that, I could set the light for a lamp, make copies, put it around the map and my players would see it with their vision set up in the same fashion, now that to this seemingly broken Update, every time I load the page, none of it was saved and I have to reset it manually. I'm assuming that's not how it's supposed to work, but I'm following the steps given by the tutorial and that's what I'm getting.

That doesn't sound like regular behavior. I have been converting a lot of games to UDL lately, and haven't run into this issue. Are all pages set to UDL as well as the tokens?

Yes, I do all the set up for tokens, light sources and map itself, and it works for as long as I keep it open, but as soon as I switch maps or close the window, it'll revert back to have no dynamic lighting.

Something strange is going on. Some things to check: 1) You do not have an extension running that might be interfering. 2) You do not have multiple games open at the same time. 3) You are using either Firefox or Chrome. (Chrome has developed a number of issues lately, but these are the only two supported browsers)

1 - I have the enhancer for shortcuts with saves and such, I tried with it off, but it still won't save. 2 - Nope. 3 - Chrome, but this has been happening since this updated dynamic light has been uploaded.

Hmm. sorry if it sounds like I'm just coming up with more things to check, but this is not a usual problem (i.e. I think there is a different root cause that just "It's UDL"). So it falls to rooting around for any other potential causes. Does this happen in all games? I.e. do the settings save in a brand new game? Have you tried in a private browsing window? (That would cut out most other potential extension conflicts, not just ones that expressly alter Roll20. People have run into issues with VPNs, some ad blockers, anti virus software and more).

Yes, all games, tho I haven't started a new one since UDL came out. Lemme check that and get back to you. Yes, private windows don't change anything, I don't use a VPN but I do have ad blockers.

Huh, new games do save, I wonder if there's a way to migrate stuff in between games.

Try duplicating your game and see if it works there. (Remember to declare which sheet you are using when you duplicate; that's a common stumbling block).