Your NAT rule seems to be correct and should do what you expected. However, there is one point to verify that I believe it is the reason why your UDP traffic is not redirected.
Redirecting UDP traffic can be more trickier than redirecting TCP. This is mainly because TCP is a connection-oriented protocol and UDP is connection-less.
NAT rules rely on connection tracking module which keeps track of the state of any "connection". In TCP case, it is clear what it means as there is special TCP packets to indicate new connections. In UDP, there is no connection establishment phase. The first packet your box will see it is considered as initiating a "connection" and the reply packet indicates that the "connection is established".
How about closing the connection? In TCP, it is also clear because TCP uses special flags to indication connection close. When seen, you box can make use of them and clear the connection state accordingly. However, you will never know when UDP "connection" is terminated. So, the connection track entry will not be cleared unless the timeout is reached (on my machine it is set to 180 seconds) without any UDP packet.
How this is related to your issue? When your box is continuously receiving UDP traffic for syslog, the UDP "connection" timeout will not expire and the connection state will be kept all the time. NAT rule will not be applied as long as there is a state in connection tracking table.
How to force new NAT rule? You can try to remove the connection tracking entry manually using the below command (don't use it without any other parameters as it will empty the whole table interrupting all your current connections). For more parameters, you can check the manual.
$ sudo conntrack -D