To make the long story short:

if i load new files into a post as attachments, then i go to click into one of the new files links just uploaded, that display into attachments panel, like so
https://localhost/phpbb33/download/file.php?mode=view&id=839
then i click back with browser to return on post, all attachments results lost (and goes into orphans).
May a sort of different behavior could be applied. Also, there are errors on console about plupload, when number of loaded attachments, are greater than the max allowed (anyway it do not cause after any issue) 

https://www.phpbb.com/community/viewtopic.php?p=15836726#p15836726