See test below - the issue occurs with Java 11 EA17 on Windows OS. The font metrics object is modified by calling g2.rotate(...).

Test-----------------------------

import java.awt.FlowLayout;
import java.awt.FontMetrics;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Rectangle;

import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.SwingUtilities;

public class RotationTest extends JFrame{
  public static void main(String[] args){
    SwingUtilities.invokeLater(() -> new RotationTest());
  }

  public RotationTest(){
    setLayout(new FlowLayout());
    JLabel l = new JLabel("123") {
      @Override
      protected void paintComponent(Graphics g)
      {
        Rectangle textRect = new Rectangle();
        textRect.setBounds(0, 0, getWidth(), getHeight());
        Graphics2D g2 = (Graphics2D)g;
        
        double origin = (textRect.width-textRect.height)/2;
        g2.translate(origin, origin);
        FontMetrics fm1 = g.getFontMetrics();
        g2.rotate(Math.toRadians(90), textRect.x+textRect.width/2, textRect.y+textRect.height/2);
        FontMetrics fm2 = g.getFontMetrics();
        assert fm1.getAscent() == fm2.getAscent();
        super.paintComponent(g2);
        g2.rotate(-Math.toRadians(90), textRect.x+textRect.width/2, textRect.y+textRect.height/2);
        g2.translate(-origin, -origin);
      }
    };
    add(l);

    setTitle(this.getClass().getSimpleName());
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    setSize(400, 300);
    setLocationRelativeTo(null);
    setVisible(true);
  }
}