Modus configuration settings are not applied

I just switched from package.el to straight. I used the following code block to download modus themes from this git repo:

(use-package modus-themes
  ;; :demand t
  :straight t
  :if window-system
  :init
  (setq modus-themes-slanted-constructs t)
  (setq modus-themes-prompts 'subtle-accented)
  (setq modus-themes-bold-constructs t)
  (setq modus-themes-headings '((t . rainbow)))
  (modus-themes-load-themes)
  :bind
  ("<f5>" . modus-themes-toggle)
  :config
  (modus-themes-load-operandi))

The themes install fine, but the configurations are not applied. For example, bold and italic constructs are not used:

Previously, I had a similar code block for installation and configuration, but I used :ensure instead of :straight. That worked fine.

It appears that the settings are set correctly, but the effect does not propagate to the buffer.

Not sure if this is a Modus issue, a straight issue, or a John issue.