Hey there!
If you're logged in, Thunder determines your post sort settings from your Lemmy account settings. If there's a bad connection, Thunder might then default back to the guest sort settings located here: Thunder Setting: General > Default Feed Type and Sorting > Default Feed Sort Type
As for the nav bar visibility, Thunder should be respecting that setting regardless of network connectivity. If you're able to reproduce this issue, feel free to open up a new GitHub issue on this so that I can take a closer look at it!