After some investigation I found out that RouterOS cuts Syslog message at 256 characters. Then add info of what module and prefix.
So the total message may be longer than 256 characters, but not the body of the message.
If I do send message to terminal using :put they are inn full length.
Here are some log message
Len 278 bytes (body 256)
script,info MikroTik: .id=*7;address=10.10.10.121;address4=10.10.10.121;age=00:00:57;board=RB941-2nD;identity=hAP_lite;interface=ether2;Bridge1;interface-name=bridge1/ether4;ipv6=false;mac-address=64:D1:54:05:82:38;platform=MikroTik;script=neighbor;software-id=76GC-9ZT5;system-
Len 284 characters (body 256)
dhcp,debug,packet MikroTik: Vendor-Specific = 00-FF-01-02-03-04-05-06-07-08-09-0A-0B-0C-0D-0E-0F-10-11-12-13-14-15-16-17-18-19-1A-1B-1C-1D-1E-1F-20-21-22-23-24-25-26-27-28-29-2A-2B-2C-2D-2E-2F-30-31-32-33-34-35-36-37-38-39-3A-3B-3C-3D-3E-3F-40-41-42-43-44-45-46-47-48-49-4A-4B-4C-
Len 258 characters (body 230)
dhcp,debug,packet MikroTik: Parameter-List = Subnet-Mask,Router,Domain-Server,Domain-Name,Router-Discovery,Static-Route,Vendor-Specific,NETBIOS-Name-Server,NETBIOS-Node-Type,NETBIOS-Scope,Domain-Search,Classless-Route,MS-Classless-Route,Auto-Proxy-Config
The two first message in green are not complete and being cut at 256 characters.
Since total messaes are longer than 256 bytes, Router OS have no problem sending longer messages.
This needs to be fixed, since it gives incomplete external logs.
This info has been posted to MT, response will be posted here.
Ticket#2019080222005241
