Ok, I enabled logging on the Wireguard zone and performed the login from TunSafe. No log entries. I then enabled the logging on the WAN zone and performed the login from TunSafe. Voila, log entries!
Sat Dec 29 21:10:00 2018 kern.warn kernel: [94382.381792] REJECT(src wan)IN=wlan-sta OUT= MAC=ff:… :08:00 SRC=192.168.1.136 DST=192.168.1.255 LEN=49 TOS=0x00 PREC=0x00 TTL=128 ID=23209 PROTO=UDP SPT=51165 DPT=32412 LEN=29 MARK=0x3f00
wlan-sta is IP 192.168.1.115, I don’t know what 192.168.1.136 is. It is not the WiFi or any LAN client. Clearly, it is a firewall issue on Slate. I did not change the firewall, simply allowed the Wireguard Server setup to set the rules.