No emails are sent (async problem – continued)

The original topic is closed so I have to open another one.
According to my tests this problem is related to capabilities of the user contributing to the forum (i.e. user who creates a new topic or replies to one). If the contributing user has administrator role then notifications are sent to the topic or forum subscribers. If the contributing user has any other role, i.e. editor or lower, no notifications are sent.
After I changed the add_action arguments ‘wp_async_bbp_new_topic’ and ‘wp_async_bbp_new_reply’ into ‘bbp_new_topic’ and ‘bbp_new_reply’ as suggested in https://www.ads-software.com/support/topic/no-emails-are-sent-async-problem/ then the notifications started to work properly regardless of the forum contributing user role.