I'm not 100% sure why that happens sometimes, but maybe it has something to do with save data persistence (when trying to save/load from a json file) on the web version being different to how it works on other versions of the game, which could potentially be what causes save data to be reset sometimes after the release of an update.
If this is the case, I may just end up using a different method for saving/loading data on the browser version that would hopefully keep the data and not reset it after each update.