Expand/collapse all only works when the data is already fetched from the server. If there are more than 500 items in the result and no filter is applied, only the top-most items are fetched.
We suggest that you filter down the result or look at part of the project at the time using the project outliner. You can also force the spreadsheet to load all data by applying a filter. A dummy filter (e.g. task state is any of the available states), can be used in case you want to look at everything.
let me know if that sorts it for you