“I’ve done a lot of copper pipe soldering in the past, but that was to the outside surface of the pipe or tube, slipping into some fitting. Never had a problem with the inside surface of a fitting, if new.”
You mean you didn’t brush or sand the inner surface of the fitting? If so, it is hard to imagine you got a proper joint.
To get a proper solder joint you always use the wire brush or sandpaper on the inside surface of the fitting just as you do the outer surface of the tubing.
And always sand the tubing a bit beyond the depth of the fitting so you get a good meniscus.
Well, it WAS years back. Maybe I sanded them and don’t recall it. Just seems like I’d have used some sort of tool for the 1/2” stuff, instead of fat fingers, and I don’t have anything like that in my ol’ plumbing tool box...
Some of those fluxes 40 years back were really aggressive, too...