I imported leads from a CSV file containing an entry under the column "Lead Source" as "List One". At the time, I had not added "List One" as an entry option for "Lead Source" in the setup options, but the leads nevertheless imported and looked correct, with "List One" as the the lead source for all entries, as expected. 

The problem arises when I try to create a view that includes all leads where "Lead Source" is "List One". I can't, because "List One" is not one of my options in the "Lead Source" field. Hoping to fix this, I added "List One" as an option using the setup page, and now I can create at least attempt to create the view. However, the view doesn't find any results.

In order to get an individual lead to show up in the view, I can go into the lead, and manually reselect "List One" as the source. This apparently selects the correct version of the "List One" entry, and now it will show up in the view. Notice that nothing appears to change: the source still shows the exact same text. 

To summarize, when I import leads with customs fields that have not yet been created under the "setup" menu, I am allowed to do it, and the fields appear correctly with the proper text. However, these new custom fields are not automatically added to the list of allowable entries, and therefore cannot be searched. When I attempt to correct this by creating a matching allowable entry in the "setup" menu, I can am then able to create a View that looks like it should work, but it doesn't. If these actions are done in this order, the View that uses the filter "lead source" IS "List One" will not return any leads imported with "List One" as the "Lead Source" IF they were imported BEFORE adding "List One" as a possible "Lead Source". It appears that if I perform these operation in the correct order (create "List One" as a value, import the leads, create the view), then it works fine. However, it should work either way, and it is too late to redo this now.