Dial app 'L' option problem

Hi everyone!

I have a problem with Dial application L option. My Asterisk 11.17.0 play the audio file one after the other (at first the A channel hear the sound file and when the A playback ended the B channel hear the sound file), but an Asterisk 13.2.0 play it well at the same time. I tested it.
I expect the AST13 behaviour from the AST11 because that is the normal. I think it is a BUG. I think the sequential operation useless in the AST11 form.

Or maybe I made a mistake somewhere?

Here is the relevant part of my dialplan:

exten => _X.,1,NoOp(Global Inbound Handler)
 same => n,NoOp(EXTEN: ${EXTEN})
 same => n,Answer()
 same => n,SET(LIMIT_CONNECT_FILE=conf-invalid)
 same => n,Dial(SIP/5001,,L(99999))
 same => n,Hangup()

If you want the improvements introduced in Asterisk 13, you must use Asterisk 13, (or backport the changes yourself, if you have the relevant programming skills).