[Android Software/Hacking General [Developers Only]] DiamondJohn:
I thought that might be the case, but I didn't wish to make an ass of me...or at least no more of...

So, on my devices I have your BusyBox Magisk module installed. I have seen that some people ROMs have implemented the "date" command, without the millisecond formatting. eg

date +"%Y-%m-%d %H:%M:%S.%3N";

returns

2020-04-05 09:52:32.3N

instead of the expected:

2020-04-05 09:52:32.421

so i checked if busybox implements the milliseconds, and it did. so it also returned (yeah, the milliseconds were different, but the point is clearer if they were the same)

2020-04-05 09:52:32.421

However, when I checked the help page, it does not show that milliseconds are supported as a formatting option. The help page returns:

ks01ltexx:/ # busybox date --help
BusyBox v1.31.1-osm0sis (2019-11-01 15:06:32 ADT) multi-call binary.

Usage: date [OPTIONS] [+FMT] [TIME]

Display time (using +FMT), or set time

[-s,--set] TIME Set time to TIME
-u,--utc Work in UTC (don't convert to local time)
-R,--rfc-2822 Output RFC-2822 compliant date string
-I[SPEC] Output ISO-8601 compliant date string
SPEC='date' (default) for date only,
'hours', 'minutes', or 'seconds' for date and
time to the indicated precision
-r,--reference FILE Display last modification time of FILE
-d,--date TIME Display TIME, not 'now'
-D FMT Use FMT (strptime format) for -d TIME conversion

Recognized TIME formats:
hh:mm[:ss]
[YYYY.]MM.DD-hh:mm[:ss]
YYYY-MM-DD hh:mm[:ss]
[[[[[YY]YY]MM]DD]hh]mm[.ss]
'date TIME' form accepts MMDDhhmm[[YY]YY][.ss] instead

While the toybox/binary on my device returns that it does support %N

ks01ltexx:/ # date --help
usage: date [-u] [-r FILE] [-d DATE] [+DISPLAY_FORMAT] [-D SET_FORMAT] [SET]

Set/get the current date/time. With no SET shows the current date.

-d Show DATE instead of current time (convert date format)
-D +FORMAT for SET or -d (instead of MMDDhhmm[[CC]YY][.ss])
-r Use modification time of FILE instead of current date
-u Use UTC instead of current timezone

Supported input formats:

MMDDhhmm[[CC]YY][.ss] POSIX
@UNIXTIME[.FRACTION] seconds since midnight 1970-01-01
YYYY-MM-DD [hh:mm[:ss]] ISO 8601
hh:mm[:ss] 24-hour time today

All input formats can be preceded by TZ="id" to set the input time zone
separately from the output time zone. Otherwise $TZ sets both.

+FORMAT specifies display format string using strftime(3) syntax:

%% literal % %n newline %t tab
%S seconds (00-60) %M minute (00-59) %m month (01-12)
%H hour (0-23) %I hour (01-12) %p AM/PM
%y short year (00-99) %Y year %C century
%a short weekday name %A weekday name %u day of week (1-7, 1=mon)
%b short month name %B month name %Z timezone name
%j day of year (001-366) %d day of month (01-31) %e day of month ( 1-31)
[B][COLOR="SeaGreen"][SIZE="4"]%N nanosec (output only)[/SIZE][/COLOR][/B]

%U Week of year (0-53 start sunday) %W Week of year (0-53 start monday)
%V Week of year (1-53 start monday, week < 4 days not part of this year)

%D = "%m/%d/%y" %r = "%I : %M : %S %p" %T = "%H:%M:%S" %h = "%b"
%x locale date %X locale time %c locale date/time

So I am guessing its coming from the source you use, and not your specific changes to build it, and, it actually functionally works.