R630, iSCSI SAN boot fail after Windows Update July rollup

Hello all, I'll try to keep this long story short but I'm not sure how much to include.

We have several Dell R630 with the same Windows 2012 R2 (!I know!) configuration and problem.
They all usually connect and boot iSCSI from a NETAPP SAN. They all use identical onboard & added 10GB QLogic 578xx NICs. 

A small group of these servers had their managed windows updates modified and soon failed to boot from the SAN.

The black & white text screen appears with the blinking cursor beneath "Press

After much trial & error (& restoring) I've narrowed it down to the installation of "2022-07 Security Monthly Quality Rollup for Windows Server 2012 R2 for x64-based Systems (KB5015874)"

Rollups prior to that do not break it. Any roll up after July (including November) will break it, so whichever patch it is, it occurred between June & July.

I have updated all the Windows Server NIC drivers. Plus the server H/W firmware, BIOS & NIC firmware to the latest I could find.

It is legacy Boot.
Internal RAID is disabled.

Dell BIOS 2.15.0

Qlogic family Firmware: 15.20.13
Controller BIOS version: 7.14.17
MBA: 7.14.2
iSCSI: 7.15.0

I've used WinRE environments (a VEEAM ISO * also Hirens) and attempted MBR & BCD repairs to no avail.

When I enable internal RAID and use the mirror as the boot system then all updates can be applied and the systems work normally. Only with iSCSI does it stop working after July's updates.
Certainly I could give in and just use the internal mirror to boot, but there are business reasons for not. Also, I WANT TO KNOW what this is lol!

The SAN fabric is 3rd party managed and I'm told there is no logging on the NETAPP that would help.

I'd welcome some ideas.
Many thanks for reading if you got this far

Andy