Can anyone help with the following problem

I am building a nested template website using Web Forms and C# (.Net 4.8) and I keep getting issues with content being duplicated in asp controls e.g the ContentPlaceholder content appears in a TextBox Control. Every now and again I will change some code on the page and it will come right for a while then with another update it will revert back to this layout

http://210.5.50.34/plesk-site-preview/christchurchrocketeers.space/login.aspx

Has an example (if it is still broken) but the images below

Screenshot of issue

Code Excerpt

Is this just an issue with the site preview of plesk and all will be fine or is there something stupid I am doing to cause the issue?

Many thanks in advance for your help

Eric

I have tried rewriting the code, excluding asp components from the code all without success