A post just made on the dev forum appeared on the RSS feed immediately (https://moodle.org/mod/forum/discuss.php?d=386762#p1559157)

I'm fairly certain that before 3.7 posts would only appear in the RSS feed after the editing period had ended. I don't know if this is a regression or an intentional behaviour change (or if I'm just mistaken about the old behaviour ) so feel free to close this if it's not a bug.