Checksums are not stored without internet access

I have this package on my system:

$ gem info minitest

*** LOCAL GEMS ***

minitest (5.26.2)
    Author: Ryan Davis
    Homepage: https://github.com/minitest/minitest
    License: MIT
    Installed at: /builddir/.local/share/gem/ruby

    minitest provides a complete suite of testing facilities supporting
    TDD, BDD, mocking, and benchmarking

This system does not have access to internet:

$ dig www.rubygems.org
;; communications error to ::1#53: connection refused
;; communications error to ::1#53: connection refused
;; communications error to ::1#53: connection refused
;; communications error to 127.0.0.1#53: connection refused

; <<>> DiG 9.18.41 <<>> www.rubygems.org
;; global options: +cmd
;; no servers could be reached

$ traceroute www.rubygems.org
www.rubygems.org: Temporary failure in name resolution
Cannot handle "host" cmdline arg `www.rubygems.org' on position 1 (argc 1)

Now trying to lock the gems:

$ cat << EOF > Gemfile                      
source "https://rubygems.org"   

gem "minitest"     
EOF

$ bundle lock --local
Writing lockfile to /builddir/b/Gemfile.lock
Resolving dependencies...

$ cat Gemfile.lock 
GEM
  remote: https://rubygems.org/
  specs:
    minitest (5.26.2)

PLATFORMS
  ruby
  x86_64-linux

DEPENDENCIES
  minitest

CHECKSUMS
  minitest (5.26.2)

BUNDLED WITH
  4.0.0.beta1

And I wonder why the CHECKSUMS section for minitest gem is empty?

Start with the reported Gemfile and reproduce the behavior using bundle lock --local without internet access; inspect how that command generates the CHECKSUMS section in Gemfile.lock. Done means the checksum behavior is explained and, if it is a bug, the resulting lockfile contains the expected checksum data with regression coverage.