Blue screen flash when unlocking iPhone

Hello,

I recently noticed that every time I press the power button to wake and unlock my iPhone, the screen shows a quick blue glitch/flash for about half a second before the lockscreen appears.

• It started suddenly while charging with my usual (but quite old) charger.
• The glitch happens every time I unlock, even when the phone is unplugged.
• Once the lockscreen is visible, the phone works normally, and no issues during use.

Has anyone experienced the same issue?

Thank you in advance for your help!