My developpers send me this info:
It works to make the acymailing plugin "system" and use it to call system triggers in our plugins, then this plugin works as a bridge between acymailing and joomla, so that everything does not have to be reprogrammed and we have not lost the settings. According to tests, this variant works.
Really this is right way create plugin bridge between Acymailing and Joomla?
I don't want to believe that version 6 is so badly programmed, the bug must be somewhere else.
Trying to have all the plugins under your control is not a good feature, I think.