I have a form library with ItemUpdated Event handler that do some specify work, I am updating the value of one of the column from form library called EmailSendToCustomer =0 and it does update the Modified datetime, it means it does execute, we have
around 1000 item in form Library.
I have a test in my local server, create a ItemUpdated event for a form library, then use PowerShell to update the form item, the event receiver can be fired as expected.
To narrow down this issue, would you please try this with a list with less items, to check whether your event receiver are fired. And also check your event receiver code, whether
SPWeb is in the correct context if it is used, it would be properties.Web.
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