No. It the max has been exceeded, entries are removed on a FIFO basis.
It's far more likely that autocompletion has just stopped working. Happens
all the time. Autocompletion is not a robust or reliable feature. It
frequently stops working usually because Outlook loses its connection to its
autocompletion cache. This can happen for a number of reasons, most often
because Outlook is not exiting completely and thus not writing to the cache.
Make sure you run no add-ins or sync software that might hold Outlook open
after you exit. After you find and resolve the problem that is preventing
Outlook from exiting properly, you may need to create a new Outlook profile
to get autocompletion to work again. If you're lucky you'll be able to
migrate your old cache to the new profile following directions here:
http://www.ingressor.com/autocompletetips.htm