Smarty SIM only works in Slot 2 OnePlus 8T

[Stevedel7]

I have a OnePlus 8T, which is a dual SIM phone.

My Smarty SIM will only work in slot 2, which is annoying because I have two Smarty SIMs and want to use both simultaneously. If I install both together, after a few minutes both slots show an 'x' in the notification bar and only clear after a reboot, and then the failure repeats.

I've tested slot 1 with an EE SIM and it registers correctly.

Can anyone please advise?

[MSF]

@Stevedel7 Looking at the OnePlus Community page, this seems a not uncommon problem and nobody suggests a solution there.