Definitely a bug but not sure why it behave like this for you, it goes in the current wiki for me on https://tuska.myxwiki.org which is a 11.5 subwiki too. Maybe you are using a skin coming from the main wiki in your subwiki ?
Search suggest (from the top right corner) retrieves the results from multiple sources that you can configure in the Administration. See https://extensions.xwiki.org/xwiki/bin/view/Extension/Search+Application#HSearchSuggestAdministration . One of these sources is the Wikis source, which is the one you are complaining about. This source gets suggestions from the WikiManager.WikisSuggestSolrService which is installed only on the main wiki ATM, so the search suggest has to make a request to the main wiki to get wiki suggestions. The main reason for this is because wiki descriptors are stored on the main wiki and thus you need to run the query on the main wiki. We could also execute the query from the subwiki, but for this we would need to change the current wiki (switch to the main one) and this requires Programming Rights, which means that if we install the WikiManager.WikisSuggestSolrService page in each subwiki there is the risk that it will be installed by an user that doesn’t have Programming Rights and thus the query won’t work.
OK I read too fast, I tough all the requests were leading to the main wiki. Indeed it make sense to ask for the wiki descriptor from the main wiki but it could be improved to make sure the current user is not lost when doing the request.