Update…Epicor Support was able to see that the emails to the DL in question were showing as “Dropped” in the SendGrid Portal and were not being sent. I found info on the SendGrid Portal that indicated once an email address is dropped it has to be deleted in order to allow future emails to be sent. On 6/5, I asked Epicor Support to delete the dropped emails but got no response until yesterday when the support person double-checked the SendGrid portal and noticed emails were now flowing for this DL. They claim nothing was done in SendGrid to delete or release these…it just magically started working 
Channeling @hmwillett
@klincecum more likely the retention policy for the dropped messages was up and once it cleared it went back to working.
This makes sense based on what I found on the SendGrid website and google…retention is 3-7 days unless an upgrade is purchased for another 30 days. Didn’t find anything specific to “Dropped” emails but was regarding Email Activity History. The last time I sent a test email was 6/1 until yesterday, 6/12 so that was more than 7 days.
Although with Epicor we do need a little 
at times!
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Update: Per Epicor Support…These records are not deleted after a certain amount of period. It could be possible that a member of the group deleted all the emails marked as bounce from the portal and as a side effect the reported issue in this case was solved.
In case a situation like this occurs once again, feel free to raise a support ticket with us in order to review if any email has been sent to the mentioned Bounce list.