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Shutdown control of MAX668 not working

Category: Hardware
Product Number: MAX668

I have a MAX668 in use for a 5V to 175V converter. The schematic is as follows:

The transformer has a 1:10 ratio, but isn't relevant to my issue.

(note, GND and GNDPWR are tied to one another)

The design was verified working on the initial prototype, but for the second prototype, the same design was used with one slight difference, the microcontroller was changed from a 5V system to a 3.3V system.
Despite no part of the datasheet indicating this should be an issue, even though I can scope the SHDN pin with a lovely clean on/off signal at 3.3V, it will not shut down

3 prototypes were made of this new design, and all 3 have exactly the same issues, component values have been triple checked.
What am I missing? 

[edited by: GenevaCooper at 3:06 PM (GMT -5) on 28 Dec 2023]
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