mktemp is a peculiar phenomenon in the linux world.
Well, I'm sure things like this happen more often, but,

Turns out, there once was an mktemp.c written for OpenBSD.
Distributions started including it, writing their own version, the usual.
Apparently, popularity of this tool became big enough, and incompatibilities
between versions arose significantly, that it was split off into its own
package and website, https://www.mktemp.org, around 2000. Written by Todd C.
Miller, the author of sudo!
This version was included in Debian under the package name "debianutils", with
the last known version being debianutils-2.8.4, where it was split into its
own "mktemp" package, both reflecting the state of version 1.5.

https://lists.gnu.org/archive/html/bug-coreutils/2007-10/msg00051.html
In 2007, GNU coreutils adds its own, slightly incompatible version of mktemp
with coreutils version 6.10.
I can only assume most linux distributions switched to it, because come 2008,
mktemp-1.6 releases, featuring GNU compatibility.
Debian switches to coreutils inmediately, while Slackware keeps it for another
decade, citing the incompatibilities, and installing the GNU version as
mktemp-gnu, only reverting this decision in 2018.

I could've sworn one of my scripts ran into the differences between mktemp-1.5
and GNU mktemp, but I can't figure out what it was, and check for it, now.