More Secure Mail Key Fun - Can't Access on iPhone but works on PC

So, my client's email stopped working on her PC and after some effort I decided to set up a Secure Mail Key (I've been down this road before...) As anyone who's done this knows, its really simple, and in a matter of minutes, her email was flowing again. I was her hero.

Next day, I get a call that she couldn't get the SMK to work on her iPhone (8 or 10, she's not sure). I stopped by and tried everything I could think of, short of deleting her account, to get this working... The SMK works great on the PC, but on the iPhone, the password fails.

Anyone seen this before or know what going on?