I would rewrite your query in the following manner:

SELECT * FROM stuff WHERE (Latitude > '29.187190582784076' AND Latitude < '29.761053992852936') AND (Longitude > '-101.0597705588786' AND Longitude < '-99.7414111838786') ORDER BY FORCE INDEX(indexedColumn) DESC LIMIT 100;

Try the FORCE INDEX option and also try the nested greater than and less than and more verbose query instead of between and check the results.

I would rewrite your query in the following manner:

SELECT *  
FROM stuff FORCE INDEX(indexedColumn)
WHERE 
    (
        Latitude > '29.187190582784076' 
        AND Latitude < '29.761053992852936'
    ) 
    AND 
    (
        Longitude > '-101.0597705588786' 
        AND Longitude < '-99.7414111838786' 
    ) 
ORDER BY DESC LIMIT 100;

Try the FORCE INDEX option and also try the nested greater than and less than and more verbose query instead of between and check the results.