Anytime a website wants to store more than 5MB of data safari asks for your permission first. This page talks about a bug in webkit where it miscalculates how much storage is being requested:

https://bugs.webkit.org/show_bug.cgi?id=202137

Some people say that deleting all the storage for the site in question in safari settings -> privacy -> manage website data will fix it. Be prepared to log in again after deleting the data.