"More" span in wrong place?

As you all know there’s <!--more--> tag.

Let’s assume we have three paragraphs:

abc
<!--more-->
defghijklmnopqrstuvwxyz

Alphabet!

Normally it outputs something like:

<p>abc</p>
<p><span id="more-[postnumber] />defghijklmnopqrstuvwxyz</p>
<p>Alphabet!</p>

But in my case (using self-made Custom Post Type) it outputs:

<p>abc <br/>
<span id="more-[postnumber] /><br/>defghijklmnopqrstuvwxyz</p>
<p>Alphabet!</p>

So it destroys the whole markup and doesn’t even show “Read more” button.

How to fix that?

Before adding (or after deleting) the tag the paragraphs are perfectly ok, but after adding – it just joins two of them and shows br’s around instead of ending the last and starting the new paragraph.

Any helping hand?

About the Custom Post Type – here’s the source (I’m using exactly the same code actually):