Compilation mode next error isn't searching all buffers

[Eric Chun]

I see the following at Compilation Mode (GNU Emacs Manual):"If the current 
buffer contains error messages or matches, these commands will iterate through 
them; otherwise, Emacs looks for a buffer containing error messages or matches 
amongst the windows of the selected frame (if the variable 
next-error-find-buffer-function is customized to the value 
next-error-buffer-on-selected-frame), then for a buffer used previously by 
next-error or previous-error, and finally all other buffers."

This seems to indicate that next-error function should switch to other buffers 
if there are no other errors in current buffer, but that doesn't seem to happen.
Here are the steps I followed:* First, I open an org file in Emacs.* I then 
create a sparse tree using a regular expression.* I then type M-g M-n to move 
to the next occurrence of my search string but it doesn't work.  I get a 
message saying "user-error: Moved past last grep hit".* I kill the pre-existing 
"*grep*" buffer in Emacs.* I then try M-g M-n again and it works.
Why doesn't next-error function jump to the next occurrence of search string 
found in current buffer?  It seems to still be looking for occurrences in the 
grep buffer.
I'm using Emacs 28.2 on CentOS7.