Still suspecting contamination as a possibility.
I have a fair pile of heavy 44 mag of various flavors going back to the late 1900's
Between moving 3 times in the past 7 years and all the funky storage conditions that go with it, this stuff is showing signs of lube migration and powder contamination.
No all of them put out all they were loaded with.
in some cases I can find a ring of unburned H110 trapped in lube where the base of the bullet was.
Fortunately, I never really cared for bunny puff loads. There is nothing with less than 10 grains of powder. even if I lose 3 grains to melted lube, I still have at least 7 on the job. Plenty to get anything out the scary end and on its way even though it'll be a bit short of design intent.

Ive been switching over to coated cast which should mitigate this issue and make loads better suited to surviving storage with grace.

I have a feeling your situation is kinda similar in that your loads check out under inspection.
you only have to lose about a grain and a half to be down into the range of a 25 autos charge range.

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it just needs more voltage