It seems I am frequently needing to re-create a search folder because it breaks and no longer can find matching emails. Going in to the properties of the broken search folder, I find that a string of characters has been tacked on to the end of the search string - which breaks it.
An example, one search folder looks for invoices. When it eventually broke, I found this in the “has the words” field:
invoice,receipt,maintenance,order,-,-,-,-,-,-,-,-,-,-,-,-,-,-,-,-,-,-,-,-,-
Note all of the ,- that gets tacked on to the end. The kicker is that even if I remove the offending characters, eM Client puts them back in. The only way to fix it is to copy all of the search strings somewhere (i.e. notepad), delete the search folder, then re-create it with those copied values (without the ,- characters). It will work for a while - until it doesn’t. 
Any ideas? Bug?