Hello @vesco27
Thank you for switching back to the AMP plugin, This looks more like a CSS issue that is related to your theme and not the AMP plugin, When you switched to transitional mode your primary theme will be served for both non-AMP as well AMP pages. but in case your theme is not AMP compatible it will look messy. which is why we recommend using Reader Mode where you can try different themes for your AMP pages.
As I checked you have switched to Reader Mode with the legacy theme
As I mentioned before you can change the look of AMP pages to something more appealing by choosing Reader Mode themes
Here are the steps to use them.
1. Install any one theme, Astra or Neve Do not Activate newly installed theme.
2. Goto AMP settings make sure you have activated Reader Mode
3. You will find the Reader Theme section under Reader Mode settings expand it
4. Scroll through the section and choose Astra or Neve theme which you have just installed.(screenshot)
5. Now click Save.
6. Goto Dashboard->Appearance->AMP to customize Reader Theme, Add logo menu other details
7. Visit your AMP page by visiting the page URL with ?amp=1
example and make sure it looks perfectly fine the way you wanted.