Sorry for the delay. We send our newsletters once per week so I needed a few weeks to make sure results were consistent. When I checked MailPoet > Settings > Advanced, we were not using the recommended setting of “WordPress built-in cron” and were instead using the “WordPress Visitors” setting. Setting it to “WordPress built-in cron” has produced consistent results over the past four weeks.
I’m running into the same issue, and I’ve seen it reported a few other times. Here is an example from our site.
Included in Email (Broken, shorter link):
https://www.wmo.org/?mailpoet_router&endpoint=view_in_browser&action=view&data=WzAsMCwxLCIwZjNjMzUiLDE3LDBd
Preview from the original email in MailPoet2 (Working, longer link):
https://www.wmo.org/?mailpoet_router&endpoint=view_in_browser&action=view&data=WzIxLCJhZDRhNGUxZGIzZDEiLDAsMCwxNywxXQ
