I know it has been four years, but in my mind the two sentences are different.
"Low-lying farmland may be at risk from flooding this weekend" in my mind means there will be flooding and this will be risky for the low-lying farmland, which may end up destroyed.
"Low-lying farmland may be at risk of flooding this weekend" in my mind means that there is a risk the the low-lying farmland will be flooded.
In another example that is more current (2020)
"Older people are more at risk from getting the coronavirus." = If older people get the coronavirus, the risk for them is higher.
"Older people are more at risk of getting the coronavirus." = Older people are more likely to get the coronavirus.
Maybe this is just old-fashioned, since I was born in 1950 and languages change.