This is the syntax for the 1.x mqtt binding. Need to use syntax for the new 2.4 binding.
Here’s a guide to help with the changes. Sonoff Tasmota with MQTT Binding 2.4 (using config files)
EDIT:
@nakh_Home: Have you created a generic Thing in PaperUI for all your mqtt things? Most recommend using PaperUI to config all mqtt Things and files for items. This helps prevent syntax errors and misconfigurations.