First: the .emacs.d/init.el test file
------
;;
;; My .emacs file
;;
(require 'package)
(eval-when-compile
(require 'use-package))
(unless (file-directory-p package-user-dir)
(package-refresh-contents))
(require 'use-package-ensure)
(setq use-package-always-ensure t)
(use-package "compat")
------
Second: 1d5b164109b59559d34c545c2a163fa067ca22b2 does work
BTW, it has to do with native compilation. I have accidentally not deleted the eln-cache in on of the tests and the 'wrong' Emacs has started correctly:
1. started 1d5b164109b59559d34c545c2a163fa067ca22b2 with eln-clean cache ---> OK
2. started b2b2be98da5825e6bc130999ffe38f5ed615586a with eln-cache from the last start --> OK
3. started b2b2be98da5825e6bc130999ffe38f5ed615586a with clean eln-cache ---> KO