I think it is null values as I suggested.

Try this amended query.

SELECT Total_x_mun.cvemun
           , Total_x_mun.municipio
           , Total_x_mun.COVID19
           , Total_x_mun.SOSPECHOSO
           , iif(Total_x_mun.total is null, 0, Total_x_mun.total)
           , catmun.shape
           , Switch(total_x_mun.total=0, 1,
                        total_x_mun.total<=10, 2,
                        total_x_mun.total<=50, 3,
                        total_x_mun.total<=100 , 4,
                        total_x_mun.total<=200, 5,
                        TRUE, 0) AS rango
           , poblacion2015.poblacion
FROM (Total_x_mun 
INNER JOIN catmun ON Total_x_mun.cvemun = catmun.cvemun) 
INNER JOIN poblacion2015 ON catmun.cvemun = poblacion2015.mungem
ORDER BY Total_x_mun.total DESC;