General snat behaviour when using multiple objects or networks as source

Hi there,

i have problems with some, but not all, ip phones not registering with an external sip provider via snat.
The snat rule looks like this:  DMZ-Voip (network) --> any --> any | SOURCE TRANSLATION: one of our public IPs.
(For the sake of simplicity, I have selected any as the destination for the time being)
The registration request goes out from 192.168.1.62, but the response comes out at 192.168.1.31.
Registration works on some IP phones, including the .31. where the mapping of the internal to the public one stays correct.
A tcpdump -vni any -s0 port 5060 shows a wrong mapping of the internal IP address on the reply:

13:48:50.003446 IP (tos 0xa0, ttl 64, id 0, offset 0, flags [DF], proto UDP (17), length 640).
    192.168.1.62.5060 > EXT.IP.SIP.PROV.5060: SIP, length: 612
    REGISTER sip:sip.provider.extern.com:5060 SIP/2.0
13:48:50.003560 IP (tos 0xa0, ttl 63, id 0, offset 0, flags [DF], proto UDP (17), length 640)
   195.xxx.xxx.67.1271 > EXT.IP.SIP.PROV.5060: SIP, length: 612
    REGISTER sip:sip.provider.extern.com:5060 SIP/2.0
13:48:50.010149 IP (tos 0x0, ttl 60, id 15929, offset 0, flags [none], proto UDP (17), length 670)
    EXT.IP.SIP.PROV.5060 >195.xxx.xxx.67.5060: SIP, length: 642
    SIP/2.0 401 Unauthorized
13:48:50.010158 IP (tos 0x0, ttl 59, id 15929, offset 0, flags [none], proto UDP (17), length 670)
    EXT.IP.SIP.PROV.5060 > 192.168.1.31.5060: SIP, length: 642
    SIP/2.0 401 Unauthorized

It works immediately when I create a SNAT rule with the single phone with higher priority. However, if I deactivate the single rule again, one would think that the registration would also fail again. But then the SNAT rule mentioned first works again.
It seems to me that a SNAT with several objects or networks does not work correctly. Can anyone confirm this?