Ok… forget muy last post. 
Bug and workaround confirmed.
Don’t tell anybody but in my head each time you wrote “issue” in my brain it was translated to “submissions”. 
I checked my “issues” table and, as you said, some issues don’t have a last_modified date.
When I change it, all works like a charm.
I will look for time to understand how the upgrade script works and make a PR, but till then, if somebody needs to fix it fast, running this query over any ojs3.x will do the job:
UPDATE `issues`
SET `last_modified` = `date_published`
WHERE last_modified IS NULL;
Cheers,
m.