I am trying to convert an 3-column edgelist / df to adjacency matrix. I've tried to convert it to a weighted graph igraph and then get the matrix using: get.adjacency() but it returns wrong weights!

An adjacency matrix will only return a 0 or 1 indicating whether a given node is directly connected (1) or not (0) to another one. Depending on whether the graph is directed or undirected, the results may differ.