While we always strive to maintain backwards compatibility of patches with older saves, sometimes changes may be necessary that prevent this. Loading an older save may not be possible on a new version of the game, or may lead to unexpected effects in game.
For this reason, we recommend that after a patch releases, you use the Rollback version available on Steam to finish your old save games before switching to the current live version. You can opt into this rollback version on the Betas tab of the game's properties, by selecting the version rollback.