The red dot means that the probe has lost its connection. measureQuick will hold the last reading for a few seconds to several minutes depending on the particular measurement. For example, return air temperatures do not change quickly so mQ holds those for a longer time.
To get fresh data, you need to be back in range of the probes.
For details on how measureQuick holds readings when probes disconnect, see Holding Measurements When Probes Are Out of Range. If your Bluetooth icon is changing colors, see Why Does the Bluetooth Icon Keep Changing Colors? For persistent connection issues, see Bluetooth Troubleshooting: Android Permissions.