When using undo-tree, the variable evil-undo-system should be set to undo-tree. The call to evil-set-undo-system does not set the variable itself; hence it used to remain the default, nil. Customizing the variable does both: It uses the custom set function that also calls evil-set-undo-system.
When using undo-tree, the variable
evil-undo-systemshould be set toundo-tree. The call toevil-set-undo-systemdoes not set the variable itself; hence it used to remain the default,nil. Customizing the variable does both: It uses the custom set function that also callsevil-set-undo-system.References: https://github.com/emacs-evil/evil/issues/1074 https://github.com/emacs-evil/evil/commit/8a3ac256804a4786bd8adbf6a3f6925162e2722f