I work in a phone shop and I'm having the same issue as a lot of people with my s9+ plus. My widgets are still disappearing a lot. Mine is the dual sim model and it doesnt happen to me every day but it is quite regular about once a month! Not only do they disappear from my home screen but they actually disappear from my phone widgets section completely as if the app isn't installed even though it is. At the moment my spotify bar type widget is missing which I use every day but it also happens with my Google search bar, weather widget and my netflix box widget 😐
I can't remember if I was running oreo or pie but i lost my weather widget, only way I could get it back was a factory reset!
A bit of effort for a widget! But I'm like that lol
I do think sometimes with the updates it's like windows on pc and you end up with corrupted files etc, factory reset is like a fresh install again.
Anyway a bit extreme for something that may or may not work bit it's all I can think of.