exact same issue here, on 4.2 *97 work perfectly, as soon as i enter a dtmf, the wav stop and process it. but *98, it process the dtmf, but after the whole wav has played... i made a trace when doing *97 and *98. *97 has this when entering a dtmf :
2018-05-07 09:07:43.768245 [INFO]...