Hello Ringtone79,
Thanks for posting in the Apple Support Communities.
If you're seeing that your iPhone display is too dark at times, check out the suggestions of the following resource to see if that helps. In addition, you may want to adjust preferences to see how this works for you like True Tone. The steps below can also be found within this resource in addition to more information: Adjust the screen brightness and color on iPhone - Apple Support
Turn True Tone on or off
On supported models, True Tone automatically adapts the color and intensity of the display to match the light in your environment.
Do any of the following:
• Open Control Center, touch and hold , then tap to turn True Tone on or off.
• Go to Settings > Display & Brightness, then turn True Tone on or off.
If those steps aren't helpful, check to see if Low Power Mode is enabled when this happens and also keep in mind that if your iPhone becomes warm it can lower the display brightness to prevent it from shutting down if it get's too hot.
Have a great day!