This is a visual issue only; it does not otherwise impact the mode.
It also doesn’t occur 100% of the time, but does happen reliably when Highforge is on the roster of Class Trial opponents (screenshot is taken from a previous week, but this week is similarly affected).
Legendaries that have shiny tokens are also “shrunk” in the soulforge. Maybe if the developers actually looked at it, they would notice that this needs to be fixed.