I have several 1911s and been shooting them for 20+ years.

I happen to have a SA 5” that doesn’t reliably lock open on the last round 1-2 out of ten times.

I’m using Wilson ETM mags and 230gr Federal hardball....

Anyone else have a gun that dos this....?

Is this an issue that can be addressed by a gunsmith and a new slide release being fitted..?

How many round/ mags through it. Is the slide release warn, what about the slide? Do you have a recoil buffer in it?  All can cause the issue.

So, a couple things, best checked with slide off.

You want to check that the follower is always under the slide stop lobe.    Some mags have enough slop that they'll allow the follower to pop over the lobe.  Or there may be a .45 slide stop (for example) installed in a 9mm gun, which needs a longer lobe.

If you can determine this, then check the plunger tension on your slide stop.  It's difficult to describe the proper amount of tension but it's not much.  Better to compare against a properly working 1911 so you can get a feel for it.

If the spring tension is too much, there are about 3 ways to reduce the tension.  Some slide stops are longer than others and depress the plunger more.  Measure yours and swap in a shorter one to test with.  This is a good option but probably the most expensive.  Alternately, you may be able to file the plunger end of the slide stop a bit shorter, but this is often not an attractive option if your SS has a finish applied, and requires basic file skills.

Second you may consider shortening the slide stop plunger.   This is easy to accomplish with a file, drill and a few minutes.   File the rounded end of the slide stop plunger flat and then chuck in a drill, and use file and emery cloth to re-round and polish the plunger.  Obviously it helps to have a few spare plungers sitting around for insurance.  I often do this on SAs because their plunger tubes usually let the plunger stick out too much, which makes it tough to reinsert the slide stop.

Third way is to clip a couple coils off the plunger spring.  Realize that this will commensurately reduce plunger engagement on the safety, which can be problematic if your safety already has soft engagement.   Often the least desirable option, but probably the cheapest and easiest, and sometimes it's the right solution.

Usually a new slide stop can fix the problem, but I've also had problems with specific mags in certain guns.  Most often my SA guns as a matter of fact.  I've had a couple of Wilsons that don't always catch and lift the slide stop, but the Chip McCormick mags I have do, so I just try to keep the Wilsons for the other guns and use the McCormick mags in my SA guns.

Known issue with Springfield and Wilson's. Wilson covers it in their FAQ page.

I’ll check the dimensions with slide off when I get some free time.

What mags are most reliable in Springers...?

I look at the mags first, sometimes it's a worn  follower or weak spring. If you can rule out the mags and ammo, then yeah look at the recoil spring next, might be weak. Look at the slide catch, might be worn.

I've had good luck with Chip McCormick's, also ACT mags seem to work okay in mine.