This topic pretty much illustrates the issue - https://www.phpbb.com/community/viewtopic.php?f=556&t=2424226

If an inline attachment is placed within formatted text (any formatting, apparently), like this:

[b]Some formatted text
[attachment=0]some attachment[/attachment]
some more formatted text[/b]

It displays like this:

*strong text*Some formatted text
[attachment=0]some attachment[/attachment] (the actual attachment does not appear, just the code)
some more formatted text*strong text*

Actual attachment appears at the end of the post, as though it were not inline

It only occurs when the attachment is within a beginning code (e.g. [b]) and an ending code (e.g. [/b].