I tried explicitly forcing emacs-internal as the coding-system for
load_source_file, but without any noticeable difference.
The button preceding the warning messages should be a stop sign emoji.
Using the built emacs to view emacs-lisp/warning.el shows the stop
sign emoji clearly, but it is apparently mangled in the generated
byte-code in warning.elc.
I also built a clean emacs from commit
2bdcf0250acecdb0719203ae711aedf5baad1783.  When I compared the source
elisp files (after building) I see differences in these files:
lisp/international/uni-bidi.el
lisp/international/uni-brackets.el
lisp/international/uni-category.el
lisp/international/uni-combining.el
lisp/international/uni-comment.el
lisp/international/uni-decomposition.el
lisp/international/uni-old-name.el
lisp/international/uni-name.el
lisp/leim/leim-list.el
lisp/leim/ja-dic/ja-dic.el
lisp/net/tramp-loaddefs.el
lisp/textmodes/reftex-loaddefs.el
lisp/loaddefs.el
lisp/finder-inf.el
lisp/cus-load.el

In the byte-compiled files, I see differences for:
lisp/emacs-lisp/eieio-opt.elc
lisp/erc/erc-nicks.elc
lisp/erc/erc.elc
lisp/eshell/em-cmpl.elc
lisp/gnus/gnus-group.elc
lisp/gnus/gnus-srvr.elc
lisp/gnus/gnus-topic.elc
lisp/gnus/gnus-sieve.elc
lisp/gnus/gnus-search.elc
lisp/gnus/message.elc
lisp/gnus/nnfeed.elc
lisp/gnus/nnml.elc
lisp/gnus/nnimap.elc
lisp/gnus/nnagent.elc
lisp/gnus/nnmaildir.elc
lisp/gnus/gnus-start.elc
lisp/gnus/gnus-sum.elc
lisp/gnus/nnmairix.elc
lisp/gnus/nnselect.elc
lisp/net/tramp-cache.elc
lisp/net/tramp-cmds.elc
lisp/net/tramp-gvfs.elc
lisp/net/tramp-sh.elc
lisp/obsolete/nnir.elc
lisp/org/org-agenda.elc
lisp/org/org-lint.elc
lisp/org/ox-ascii.elc
lisp/org/ox-odt.elc
lisp/org/org.elc
lisp/progmodes/eglot.elc
lisp/progmodes/cc-engine.elc
lisp/progmodes/cc-mode.elc
lisp/progmodes/js.elc
lisp/use-package/use-package-core.elc
lisp/loaddefs.elc

I'm not clear on why loaddefs.el should be different.  That, and
eieio-opt.elc being different is unexpected.

Lynn