It appears in our internal installation of Portal for ArcGIS 10.5.1, the keyword search box in the top right does not allow for either partial word searching or wildcard searching. If you try to use a wildcard (*) you get a message saying Wildcards aren't supported. If you search for part of a word, for example mex for mexico, you won't find anything that isn't actually named "mex" or tagged "mex", etc. At least it doesn't seem to be case sensitive which is a blessing I suppose, but what is so hard about wildcard or partial word searching?
