Is it normal for Ecoserver.exe to forget SLG Login Info after an update?

The token is kept through updates, but based on the path to the server so you can host multiple ones. Means if the server after the update has a different path to EcoServer.exe (because you for example just unpack the .zip file from play.eco that now has a different folder name), it will have forgotten its cached token. If not, login should have been remembered.