Interesting - thanks for persisting.

When I revert to an older Strawberry Perl that wasn't built with -D__USE_MINGW_ANSI_STDIO (5.24.1 / MSWin32-x64-multi-thread) I get the same as on Ubuntu:

C:\_64\strawberry5.24.1>perl -e"CORE::say sprintf '%.20g', 0.1"
0.10000000000000001

C:\_64\strawberry5.24.1>
[download]

I don't think that there's a lot of significance in the output of '%.20g' formatting of doubles.
As you're probably aware, both 0.10000000000000001 and 0.10000000000000000555 will assign to the same double anyway.
Both a 20-decimal-digit value, and that same value rounded to 17 decimal digits, will assign to the same double (unless something is buggy).

If the intention of "%.20g" formatting of 0.1 is to get the closest 20-decimal digit approximation to the double 0.1, then the latter is correct, and the former (ie the value consistently provided by Ubuntu) is incorrect:

C:\>perl -MMath::MPFR=":mpfr" -le "$x=Math::MPFR->new('0.1'); $base=10
+; $digits=20; Rmpfr_out_str($x, $base, $digits, MPFR_RNDN);"
1.0000000000000000555e-1
C:\>
[download]

Perhaps Linux has some catching up to do.
Even VC++ 2017 and later are "getting it right". On perl-5.30.0, MSWin32-x64-multi-thread, built with VC++ 2019:

C:\>perl -e"CORE::say sprintf '%.20g', 0.1"
0.10000000000000000555

C:\>
[download]

I'm not exactly sure when VC++ started "getting it right". I know it was some time after VC++ 2010.

Cheers,
Rob