This appears to have been resolved in the past seven years.  I was testing this to look at possibly working on it but it appears to be a moot point.  My testing:

log into opac with account of home lirary of Centerville, load bib with three items:
333 - centerville home lib
4444  - centerville home lib 
55555 - fairfield home lib 

system preference:
OpacSeparateHoldings - separate based on home library 

opac details: 
Centerville Holdings (2)  Other Holdings (1)

add 4444 to course reserves with Fairfield as new home lib; leave holding lib unchanged 
opac details: 
Centerville holdings (1) 
Other holdings (2) - 4444 now a home lib Fairfield item has moved over to the Fairfield tab 

remove item from course reserves
everything resets 
add 4444 setting holding library to Fairfield but not home library 

opac details:
Centerville holdings (2) - including 4444 with current location of Fairfield Fiction but home library of Centerville, behavior correct based on system preference 
Other holdings (1)

remove item from course
re-add with both home and current library changed to Fairfield 
item moves to Fairfield tab as expected 

change OpacSeparateHoldings to don't separate 
everything comes back to one tab correctly

change OpacSeparateHoldings to separate on holding library 

without going into details I tested the above again with the variant of looking for holding library and it behaved as expected

all right, I had a brain failure and didn't grok that the ticket was about the course reserves column in the details column, I can confirm the current behavior is still as reported

This is a really old one...

For what it's worth, I can't get the course reserves to appear on opac-detail.pl even without OpacSeparateHoldings...