Weird "_dateparsefailure" issue

Hi all,
I'm facing a really weird issue parsing the date from an access log that doesn't use standard ISO8601 format but one that looks like: [08/Sep/2020:18:39:23 +0200]

I captured the group with a simple grok in a field called "t_stamp" and then applied this date filter:

date {
    match => ["t_stamp", "dd/MMM/yyyy:hh:mm:ss Z"]

Problem is that this works only in about 1/3 of the logs, the rest are indexed with a _dateparsefailure error:

Log correctly parsed

Log incorrectly parsed.

Since all the strings, option and fields seems the same I really cannot understand why this error. Any clue?

hh is the clock hour of the half day (1 to 12). You should use HH.

Oh, You're right, now everything works as expected.
I can't believe having fell for such a silly error.