Angular material previous step's number icon changes to characters

[gentle ravine]

as the image provided, number 1 changes to what appears to be 'cre'

[dusky dust]

that's not a character but it's trying to display a mat-icon which I suppose you're not using in your project

to disable the icon I believe you could add to the step

 <mat-step [editable]="false">
   Step One
</mat-step>