Some guys will install two or more adapter bushings, into a coolant port on the intake, to make their aftermarket parts fit together. This raises the sensor well out of the coolant's flow-path and is the equivalent of a short pipe.
With the sensor thus in stagnant water, inside a pipe that is radiating it's heat into the surrounding air, it does not properly indicate the temp of the water that is flowing just an inch or so away. The difference can be dramatic.
Add to that, say, a brass, or iron plumbing bushing, in an alloy manifold, and there's another variable to deal with.
And then there's grounding.....
IMO, weld up the hole like you and others have suggested.
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I have this other opinion, namely; trying to tune an engine in the which the coolant temp is ever-changing, like say with electric fans, is an exercise in frustration. I imagine, tuning an engine to an erroneous coolant temperature, might also be frustrating.
Plus, let's say your coolant is at 210, but the gauge is reading 190. You might be lulled into thinking that everything is ok. But, you have no temperature headroom, if for some reason the coolant-temp actually rises 10 or 20 degrees.
Or how about, when your fuel starts to boil on a hot day, what's the first thing you do? IDK about you, but Ima checking my temp gauge. If it's telling me 190, Ima thinking that's fine and I start dissecting the fuel system, maybe wasting hours on that, until I recheck the daymn coolant temp. and the liquid is actually much hotter.
How do I charge my customer for the hours I wasted?
I don't, I learned my lesson;
Put the sensor into the moving water, and verify the gauge with an IR gun. Then make sure the gauge is reliably consistent.
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One of my pet peeves is that guys(streeters) will spend outrageous amounts of time and energy
1) trynta run 20 degrees of idle-timing, or
2) the last two degrees of Power-timing, or
3) looking for 1psi of fuel pressure, and
4) their engine is inhaling hot underhood air,
5) yet, they don't spend hardly any time studying their coolant temp. until the engine overheats and pukes it out.
end rant.