When you start a new instance of minecraft and join on a server, autoswitching will be disabled.
If you press the assigned key, it will stay on "Disabled".

Known workaround:

1. Load into a singleplayer world and press the key three times till it shows "Enabled".
2. Go back on your server and press the key until it shows "Enabled".

Minecraft version: 1.15.1
Fabric version: 0.7.3+build.176
Fabric api version: fabric-api-0.4.26+build.283-1.15
mod version: autoswitch-0.8.0

Fixed in 0.8.1
https://github.com/dexman545/Fabric-Autoswitch/releases/tag/0.8.1