Magento 2 Multi-Store – Multiple Stores with Different Root Categories

My first guess is, that magento can't see, that you are surfing on your new store, instead of your old store under the category nested-store/about. Therefore the first thing you have to check is, whether magento uses the right storeview here.

If this is not the case, then you should add a vhost configuration wich sets the right website/storeview, like this:

SetEnv MAGE_RUN_CODE {storecode}
SetEnvIf Host www\.domain2\.com MAGE_RUN_CODE={storecode}

or

SetEnv MAGE_RUN_CODE {websitecode}
SetEnv MAGE_RUN_TYPE website

If this is the case, the only idea I have is to dig into the routing with xdebug or any other debugger of your choice and check what happens inside.

And a side note: Having a store in a subdirectory of another store might count as "a bad idea" :-)

When new Website or store view created you have to run the following command in your commandline,

php bin/magento indexer:reindex

Once this done, you can able to see your new store in design section to edit and set the custom theme.