Customize option read-buffer-completion-ignore-case to non-nil.
C-h v read-buffer-completion-ignore-case tells you:
read-buffer-completion-ignore-case is a variable defined in C source code.
Its value is nil
Non-nil means completion ignores case when reading a buffer name.
You can customize this variable.
Note, that I assume that isearch-toggle-case-fold is not supposed to be run outside of isearch-mode.
Cause of the effect:
The highlighting belongs to the overlay stored in variable isearch-overlay generated or moved by function isearch-highlight.
isearch-highlight is called in isearch-update which is called in isearch-toggle-case-fold generated by isearch-...