I encountered this once on my Android device as well, but I’m not sure how to reproduce it.
What I remembered was that:
Using my current loadout, I located the Girros Charge Blade that I have already forged and opened the Style Customisatio menu and entered the training room with it without equipping it to the current loadout.
I tried out both the Sword Boost and Axe Boost styles in the training ground, then I left the training ground.
I switched to another loadout, then again opened the Style Customisation menu for the Girros Charge Blade and entered the training ground from there.
At then, I found that I cannot see the style selection menu from within the training ground.