Have you updated the game to 11.2? This bug happened when two idols had a birthday on the same day.
Can you please upload your save file to https://file.coffee if you still have it?
Windows:
C:\Users\*username*\AppData\LocalLow\Glitch Pitch\Idol Manager\data\auto_save.json or
C:\Users\*username*\AppData\LocalLow\Glitch Pitch\Idol Manager\data\manual_saves\*\save.json
Mac:
~/Library/Application Support/Glitch Pitch/Idol Manager/data/auto_save.json
~/Library/Application Support/Glitch Pitch/Idol Manager/data/manual_saves