Does anyone have any idea why a widely used ISO 100 diester compressor oil would have isopropyl alchohol in it?

Is this put in as an additive and if so for what purpose?

IPA may be used as a cleaning solvent when switching between products.

Hi Swampdiver
The usual method of preparation of esters is reaction of an acid with an alcohol,
the IPA is very likely to be part of the original formulation,
we use esters for their seal swell charateristics to counter act the seal shrink in the POA solid boundary EP gear lubricants we manufacturer,
in high temp application the correct esters, there are thousands from food flavourings to lubricants and others, the correct ester will resolubilise any heat degraded base lubricant preventing any hard lubricant deposits

regards Rob S