Asterisk 13 DTMF choosing wrong codec

Hi all,

Currently running a pretty old version of Asterisk, we are due to upgrade in the near future but just need to find a way around this issue in the meantime on the current version 13.10.0

We have a system setup as
PSTN => Asterisk PBX => Asterisk PBX => PSTN

Calls work fine when going out and use PCMU both ways

sip.conf

disallow=all
allow=ulaw,alaw
dtmf=inband

Changing ONLY DTMF causes Asterisk to send PCMA and receive PCMU; the priority is the same in the sdp with PCMU being listed first

disallow=all
allow=ulaw,alaw
dtmf=rfc2833

Is this a bug that got fixed at some later version? I have never known DTMF to impact codec choice or cause a different codec than specified to be sent.

In the meantime are there any options which can assist in this?

Thanks in advance!

EDIT:

Tried restricting to use just ulaw.
Still send alaw when DTMF is rfc2833
Send ulaw when DTMF is inband