I confirm this bug in the latest build of emacs-25 branch.
I can confirm that evaluating below:
(setq minibuffer-prompt-properties '(read-only t point-entered minibuffer-avoid-prompt face minibuffer-prompt))
"does the right thing" in the minibuffer in emacsen 24.3, 24.4 and 24.5.
That is, arrow key navigation and C-x h does not enter the part of the minibuffer that should be read-only.
To test it,
1. emacs -Q
2. C-x C-f
3. Pressing left arrow key will take the cursor to the left most area within the read-only area "Find file:"
4. Now evaluate "(setq minibuffer-prompt-properties '(read-only t point-entered minibuffer-avoid-prompt face minibuffer-prompt))"
5. C-x C-f
6. Now the cursor will stop to the right of the colon in "Find file:" when the left arrow key is kept pressed.
Note that above works as expected in emacsen 24.3, 24.4 and 24.5.
But it does NOT work in the latest build of emacs-25 branch. On emacs-25 build, instead of Step 6 behavior, we see Step 3 behavior. Setting point-entered does nothing.