By default, Zoe displays the rank of the last game queue per player. This means that if their last game was a ranked flex, this change will be displayed.
You can change this behaviour in Zoe's configuration as follows:
Use /config to get into Zoe's configuration
Select the option for 
Info Channel Settings
Click on the corresponding setting 
Infochannel Rank Shown in the following menu.
The next thing you will see is the configuration message with a description, a list and buttons.
Click on the list and choose SoloQ.
After that continue with these settings to finish the configuration.