Notification for following disappear after upgrade

[Bot]

I have recently upgraded from 4.6 to 4.17 and some of my members are reporting they no longer receive notifications for topics and users they are following. Unfortunately, it seems that the problem is not easily reproducible, for some topics it works, for others it doesn't, some users do not appear to have the issue, others are more affected. 

They mentioned that it used to be sporadic even before the upgrade, but now after the upgrade it is much more noticeable. 

I know this is pretty vague, but any idea what might be wrong, is it server side or client side?

 

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