as from the comment:

it seems that we have only 0-4 that could be used as non-error levels, and 0 as the default(log.Info() without V()).

this makes all the non-error as default printed levels, which leads to printing all Debug levels.

if I increase the level to 1-4, I will lose all the default Info() level logs.

this really confused me, may I ask how you use the level to switch different level detailed logs?