After a bit more digging I discovered that OSN plugin is making some changes specifically for SnowFlat theme, in order for the bell to appear next to the User Avatar at the TopBar.
So if you're using a Modded version of Snowflat theme, with a new different name, for example 'SnowflatCustom', these SnowFlat targeted changes won't take effect.
To fix this you'll need to add your SnowflatCustom to the targeted changes as well.
Got to - On Site Notifications > q2apro-onsitenotifications-layer.php > line 122
Change that line from this:
if(qa_opt('site_theme')=='SnowFlat'') {
to this:
if(qa_opt('site_theme')=='SnowFlat' || qa_opt('site_theme')=='SnowflatCustom') {
Most of Modern themes already use that style of User Navigation dropdown (
https://i.ibb.co/9VMxz7z/z317.jpg), so I could push a better solution to output the bell to the official repository. But with that fix, older themes may appear displaced. So I need to think thoroughly of a more general solution, or use that same logic of line 122 but instead of targeting SnowFlat, target all the old themes available... Will see.