this might be really simple.
My config has
payload_low_speed: "speed=1"
payload_medium_speed: "speed=2"
payload_high_speed: "speed=3"
what if your config was set to:
payload_low_speed: "VAR1=1"
payload_medium_speed: "VAR1=2"
payload_high_speed: "VAR1=3"
Can you post your whole HA side config? I’m really not great with tasmota myself and mqtt can also mess me up pretty easily however it might be something obvious