ICX Stack upgrade problem

Working on what should be a very straight forward task but becoming a nightmare.  We have some ICX-7450 stacks (a 2x and 5x member stack).  The task is to upgrade from 08.0.30t to 08.0.90a.   Thought was to tftp the 08.0.90a code to the secondary along with tftp’ing over the new 10.1.15T boot loader.  The TFTP’s complete successfully and a sh flash shows what one would think is a working configuration.  Reboot the stack with the ‘boot system flash secondary yes’ command.  Both units in the stack come up successfully.  Here is where the frustration starts.  Unit 2 reboots due to a stack election (not totally sure why the election occurs when the stack is already been working).  Upon this second reboot the boot load reverts back to 10.1.06T and then gives a invalid bootm command and won’t load the 08.0.90a code.  I have tried to update the uboot with the bootm functionality. Works once then reverts back again.  So I end up rolling the secondary back to 08.0.30t to get the stack stable again.  I then try the same process but using the primary. Again upgrade seems to work but ultimately a unit in the stack reboot and reverts back to older bootloader and has bootm & kernel errors.  I can upgrade stand-alones all day long no issues,  but stacks just aren’t liking the 08.0.90a upgrade for some reason.  Anyone else seen this or have some suggestion to get past this?  Thank you!