The warning may appear starting from 7.17:
If you see the log message right after reboot (as it is the case from your screenshot), and not later, then you can ignore it.
While your router reboots / starts up, there is period where it's not available. If clients in your LAN have the router configured as DNS resolver (it's the case in your config, with the /ip dhcp-server network entry specifying dns-server=172.17.0.1), then the clients might still make DNS queries while your router reboots.
First they all try with UDP, it doesn't work because the router doesn't respond. After a brief period (a few seconds) the clients will try to contact the DNS server (your router) over TCP, they all will try to initiate a TCP handshake to port 53 of the router, each might try multiple times, by sending TCP SYN packets.
You'll then have a situation when right after the router finished rebooting, it sees a bunch of TCP SYN packets arriving on port 53, probably from multiple sources (clients). This will cause the warning message above to be logged.
In normal router operation, you'll see fewer of such messages, because most clients will be able to make the queries over UDP, and only need to fallback to TCP when the responses are too large. And also because normally the queries succeed and are answered on-time, so clients don't have to retry and switch to TCP.