I've looked into this change a bit - because it also happens on another forum that I am active on. Checking over there, I found a reference to this bug report on phpBB 3.2: https://tracker.phpbb.com/browse/PHPBB3-14754
In reading through the comments on this bug report, it seems that there is no fix for those of us who like to use e-mails rather than the notification center to monitor activity in subscribed topics, except to turn on both board notifications and e-mail and then ignore the notification center.In 3.2 it is possible to be choose to be notified only by email.
When choosing to be notified only by email, subscribing to a topic sends an email for every single reply to that topic. This is far too much email spam and contradicts what the email says it will do.
When choosing to be notified by both email and board notification, you will receive an email and won't receive a new email until you visit the topic.
I have turned on board notifications tonight, and as expected, I am now receiving only one e-mail per topic.
Thought I would share in case others have experienced this as well.
MaryAnn