In the templates:

• Template Module Linux network interfaces by Zabbix agent
• Template Module Linux network interfaces by Zabbix agent active

 The trigger "Interface {#IFNAME}: Ethernet has changed to lower speed than it was before" isn't working as it's not using the interface speed.

There is also no "Interface {#IFNAME}: Speed" item prototype defined which can be set to   "vfs.file.contents"/sys/class/net/{#IFNAME}/speed"" which should contain the current interface speed.