Wyse 3040 Downgrade 9.1 to 8.6 and Secure Boot

Hello

I'm having to downgrade a bunch of Wyse 3040's back to 8.6_710 due to issues in 9.1 (another thread). I've found that I cant boot into the USB imaging tool without turning off secure boot. Once imaged, turning secure boot back on stops the thin client booting.

Is 8.6_710 not compatible with secure boot or is there any risk leaving it off?

Thanks