Making this post because I've seen a lot of people comment now that they're having the same issue.
Friendly advice, ignore Kurtosis12, he doesn't know what he's talking about and most likely an apple fan crying because someone said apple did something wrong.
The only thing we can really do while we have this problem is wait for apple to roll out an update that reverts the change they made probably with the increased the customisability for the bottom lockscreen buttons that has broken widgets for many apps, not just snapchat. Or wait for snapchat to release an update that makes their widgets compatible for whatever change IOS 18 has introduced. Unfortunately, either way is just a waiting game.
And Kurtosis, You don't need to come back and comment the same thing for a fifth time now, snapchat support does not have a channel for this issue, there is no use in contacting them because they only have automated support lines which do not cover this. Please find a better way to spend your time, maybe touch some grass.
Hope everyone who is having this problem finds this helpful.