Hidden "Place inline" button

Hello,

I'm creating a new post, then use the attachments tab, I upload a very small .jpg or .png file (10kb, 40x40), the image line appears but the "Place inline" button stays hidden.

When I'm trying to do the same in a post that already contains images, I get the same behavior, but the lines with the previous images do contain the "Place inline" button.

Looking at the HTML code, I see that :

<input type="button" value="Place inline" class="button2 hidden file-inline-bbcode" />

This phpBB forum is the production one, and I have an exact copy with the same user, the same permissions, the same settings about the allowed sizes, php.ini settings, and so on.

The account I'm using has the admins perms. I also tested it with a basic account, to no avail.
Though, some of my users are using it with no issue.

This is driving me mad because I find no difference between the QA and the prod instances.

What should I look to debug this further?

Thank you