Fax calls hanging up prematurely

[rhuntertec]

IPO Preferred Edition 11.0.4.4
Using Grandstream HT814 ATAs to power the fax machines on the system.
Not every time but lots of times, during the middle of an outgoing fax call, the far end fax machine will hang for no apparent reason.

15:29:37 517061355mS ISDNL3Evt: v=1 stacknum=1 State, new=ReleaseReq, old=Active id=37600
15:29:37 517061362mS ISDNL3Evt: v=1 stacknum=1 State, new=NullState, old=ReleaseReq id=37600
15:29:37 517061362mS CMLineRx: v=1
CMReleaseComp
Line: type=Q931Line 1 Call: lid=0 id=37600 in=0
BChan: slot=0 chan=23
Cause=16, Normal call clearing
15:29:37 517061362mS CMCallEvt: ac19dc02000092e0 0.37600.0 7471 Q931 Trunk:1 CHAN=23: StateChange: END=B CMCSConnected->CMCSCompleted
15:29:37 517061364mS CMLOGGING: CALL:2021/01/2715:27,00:01:34,009,8371,O,14172066276,914172066276,PubAdminFax,,,0,,"",0,n/a
15:29:37 517061366mS CMExtnEvt: PubAdminFax: CALL LOST (CMCauseNormal)
15:29:37 517061366mS CMExtnEvt: PubAdminFax: Extn(8371) Calling Party Number(8371) Type(CMNTypeInternal)
15:29:37 517061366mS CMCallEvt: ac19dc02000092dd 402.37597.0 -1 PubAdminFax.0: StateChange: END=X CMCSConnected->CMCSCompleted
15:29:37 517061366mS CMExtnEvt: v=0 State, new=PortRecoverDelay old=Connected,0,0,PubAdminFax
15:29:37 517061367mS CMExtnTx: v=8371, p1=0
CMReleaseComp
Line: type=SIPLine 260 Call: lid=402 id=37597 in=0
Called[914172060000] Type=Default (100) Reason=CMDRdirect Calling[8371] Type=Internal Plan=Default
Cause=16, Normal call clearing
Timed: 27/01/21 15:29
15:29:37 517061368mS SIP Tx: TCP 172.25.220.2:4105 -> 172.25.220.224:5060
BYE sip:8371@172.25.220.224:5060;transport=tcp SIP/2.0
Via: SIP/2.0/TCP 172.25.220.2:5060;rport;branch=z9hG4bK8021af654adab9512a9dd8fbe803d780
From: <sip:914172060000@172.25.220.2>;tag=8c120e9955636c12
To: <sip:8371@172.25.220.2>;tag=1723121919
Call-ID: 1175556401-5060-21@BHC.CF.CCA.CCE
CSeq: 192 BYE
Contact: <sip:914172060000@172.25.220.2:5060;transport=tcp>
Max-Forwards: 70
Allow: INVITE,ACK,CANCEL,OPTIONS,BYE,INFO,REFER,NOTIFY,SUBSCRIBE,REGISTER,PUBLISH,UPDATE
Supported: 100rel
User-Agent: IP Office 11.0.4.4.0 build 6
Reason: Q.850;cause=16;text="Normal call clearing"
Content-Length: 0


It will be in the middle of sending to an outside phone number or even another fax extension on the system and the far end just initiates a BYE or a Normal Call Clearing despite the sending fax not being finished.
It's not tied to a specific number of seconds or a timer that I can find, usually happens within 40 to 100 seconds but not always. Happens on most if not all of the 13 ATAs I am using.
No packet loss or jitter. T.38 version 0, max speed 9600, ECM turned off.