Bug overview description.
Quoting complex comments makes following text invisible

What part(s) of the site page(s) are affected?
Comments on posts

What is the expected behavior?
The text being visible is the expected behavior

What actual behavior is given instead?
The text is invisible

Time of incident (if applicable).
N/A

Can you reproduce the bug every time?
yes

What steps did you take to replicate this bug?
1) Go to post #1066994
2) Quote the first post, and add a comment, e.g.
That song is so catchy, I must put it on my iPod(tm)
3) Click "Post"
4) The quoted part is visible, but not the new text from step 2
5) Yes, even after you reload the page

I even made a test post to verify this:
https://e621.net/comment/show/2948199
It has the text "this is only a test" after the quote, but the new text is invisible.

never mind, the new text somehow becomes part of the quote

Errors or other messages returned (if any).
no error messages

It gets added to the Click to expand section. Try adding \[/section] at the end of the quote.

It's not invisible; your text just gets stuck inside the section. Because whoever wrote it forgot to close it.

So you need to fix the formatting yourself when replying to those.

Genjar said:
It's not invisible; your text just gets stuck inside the section. Because whoever wrote it forgot to close it.

So you need to fix the formatting yourself when replying to those.

Then it would be very nice if e621 checked for unclosed (and unopened) DText tags.
I'll make a suggestion post.

Munkelzahn said:
Then it would be very nice if e621 checked for unclosed (and unopened) DText tags.
I'll make a suggestion post.

We already have one: forum #203070
May need to pester Kira to reopen it.

Furrin_Gok said:
We already have one: forum #203070
May need to pester Kira to reopen it.

That specific bug was fixed. This is a case of the raw markup being poorly constructed and your content getting caught inside it. Since you're just providing text directly to the server and the clients don't know how to parse DText, this turns into a fairly complex issue. Since neither side knows that when combined it produces the wrong output.

Automatic output closing is a bit of a requirement in DText, but as a feature, is a mistake, because it leads to this.