How do I get objects in ursina to rotate around a consistent axis?

Question

Pressing the keys r and u rotate a cube around the x and y axis respectively. When pressed individually, the rotations are fine however when U then R is pressed it rotates it around a different axis. How do I keep the axis consistent.

Link to model and texture if you want to run the code: https://drive.google.com/drive/folders/1R598KCxo3EbbQIcuzeQX1wYKlu1DJYoV?usp=drive_link

from ursina import *

app = Ursina()

origin = Entity()
rotation_origin = Entity() # creates an empty entity that the camera orbits around

camera.parent = origin
camera.z = -50  # sets initial position of the camera

cubelet = Entity(model='models/cubelet', texture='cublet texture.png', position=Vec3(0, 0, 0), rotation=(0,0,0), scale=(4,4,4))
cubelet.world_parent = rotation_origin

def input(key):
        if key=='r':
                rotation_origin.rotation_x += 90
                print(cubelet.rotation)
                
        if key=='u':
                rotation_origin.rotation_y += 90
                print(rotation_origin.rotation)

def update():
        origin.rotation_y += (mouse.velocity[0] * mouse.left *100)              # allows for the camera to be moved by the mouse
        origin.rotation_x -= (mouse.velocity[1] * mouse.left * 100)


app.run()

This is part of a bigger program of a 3D model of a Rubik's cube and I am just trying to get the individual pieces to rotate how I want them to.

I have tried anchoring it to an origin, I have tried it using only the cubes axis but still got the same result.