You are using an article on the old Wiki. Not only MikroTik staff but other users could also write articles on that Wiki and the article in question is not from MikroTik but from some random guy.
Here is the current official firewall example from MikroTik:
Please note that not everything mentioned in the new article is relevant and recommended. For example, all those RAW rules can be ignored.
As for the reason why what you posted above doesn't work and cause problem: Study this flowchart of the MikroTik firewall:
You can see that srcnat happens at the end of the flowchart, on step #25. Which means for the first packets of a new connection being forwarded, when the filter rules are checked on those packets on step #14, there is absolutely no information that the connection will be SRCNATed yet. Which means your check for connection-nat-state=!srcnat always yields true and considered "matched". Thus, packets of a new connection from LAN to WAN will all match the action=drop rule because all conditions are true (src-address-list, out-interface-list, and connection-nat-state=!srcnat).
My recommendation is to undo whatever that questionable Wiki page suggested (the whole SYN flood and port scan checks are frankly useless), go back to the defconf firewall or implement what suggested in the new documentation (minus RAW rules).