Just to let you know it looks like I've found a minor bug. I recently updated a survey and its associated thumbnail for the mobile app version of S123.
It looks like the survey itself updates fine (phew) but the update to the thumbnail isn't pulled through. Users have to delete the form from the device and then download it again to see the new icon.
What version of the mobile app are you using? I am not seeing the behavior you described using the latest 3.11 production version and 3.12 beta version of the field app. Could you provide the version of the field app and the mobile device model you are using?
Could you list the model of the mobile device you are encountering the issue? The 3.11.164 is the production version and I was not able to reproduce the same as you described. A detailed info about the device model and OS version would be helpful to narrow down the issue here. And did the same issue occur on different surveys or only a specific survey?