What version of Hugo are you using (hugo version)?

hugo v0.115.3+extended darwin/arm64 BuildDate=unknown

Does this issue reproduce with the latest release?

Yes

When defaultContentLanguage is set to "it", using (for example) {{ dateFormat "02 Jan 06" .Date }} returns May as maggio instead of mag. This only happens with May, other months are displayed correctly.

{{ .Date | time.Format ":date_medium" }} is working correctly, returning mag instead.

Out of curiosity, I've also tested es and fr and saw some inconsistencies there too. I'm not a Spanish/French native speaker, so I can't say if this is normal behavior, but both have some months shortened as 3 letters and some others as 4 letters (May in Spanish seems to have the same problem).