Re: WIRED: different logs from diff SRX with the same config and Junos version...

"but  c'mon -I don't want to rewrite ALL filters every time I update software on SRX -this is sick..."

I still don't know for sure that the problem is from the difference in the definitions I just want to check everything because you've obsucred the IPs in the referenced logs. You are sure the referenced logs are from both SRXs right ? ( just to make sure ). Also, not to defend Juniper, but from another point of view you could say that the issue is with the Syslog server not automatically updating syslog messages device definitions ( if you were using Security Director or JSA ( Juniper SIEM ) you could get new defintions for the syslog messages as well ) .

You could also review your filters to make more generic regexs ( e.g., SNMP* ). I have a similar definition to your on one of my client devices and this is the definiton of the SNMP App ( note the Alias List section ):

root> show services application-identification application detail junos:SNMP-VERSION-2U
Application Name: junos:SNMP-VERSION-2U
Application type: SNMP-VERSION-2U
Description: This signature detects the Simple Network Management Protocol (SNMP), which is a protocol used to manage nodes (servers, workstations, routers, switches,
             and hubs etc.) on an IP network. It usually runs on UDP ports 161 and 162.
Application ID: 190
Disabled: No
Number of Parent Group(s): 1
Application Groups:
    junos:infrastructure:monitoring
Application Tags:
    risk                  : 1
    subcategory           : Monitoring
    category              : Infrastructure
Over List:
    UDP         / 216
    HPELLC      / 65
Alias List:
    junos:SNMP
    junos:SNMP-VERSION-1
    junos:SNMP-VERSION-3
    junos:SNMP-VERSION-2C
Port Mapping:
    Default ports: UDP/161
Signature:
    Port range: N/A
    Client-to-server
    Order: 1

"how it's possible to have a different engine versions for the same hardware and Junos set?"

The Application Identification Engine and definitons are seperate modules from the Hardware and OS. This is a layer 7 identification  module that is partially software based and is independent. The reason the versions are different is because they haven't been updated in sync ( you can configure them to auto-update if they have internet access ).

I would advise you to update the bad one and check if this issue is resolved or not. 

Can I ask what Syslog Solution you are using ?