Pull Up Resistor on signal too low (Tx from HC-06), why it get fixed?
It's probably an open collector output, whose circuit looks something like this:







simulate this circuit &ndash; Schematic created using CircuitLab

You have to add an external pull-up to introduce a high level to the circuit. This is a common way to make a voltage-agnostic output interface, since you're only limited by the output transistor voltage rating or any internal ESD diodes on the pin.