When running a reindexing operation for your products, the process starts by requesting the necessary product data from Shopify. Shopify then generates a bulk file containing all the product data. This process can take some time (up to 24 hours depending on the size of your catalog). During the time the bulk file is being created, the progress bar will remain at 0% (as we still haven't received any data from Shopify at this point).
Once we receive the bulk file from Shopify, the progress bar will start running to indicate the percentage of data that has been processed on our end.