I have about 17,000 customers in customer_entity that were imported from another platform. For the few months we've been on Magento 2, customer_grid_flat has updated properly and I'm able to see all customers in the admin. However *something* happened and customer_grid_flat is no longer updating. The table now consists of 92 customers - only those that have explicitly logged in for the last few days. There were no code changes done, and I've run a database compare from a backup a few days earlier where everything worked. Nothing stands out other than customers changing their passwords.
I can manually rebuild customer_grid_flat from the backup, but I assume this gets rebuilt on reindex (is that true)? Are there any other table differences I should be looking at? Anyone have any ideas?