Hi @Anonymous ,
Do you want to save the Rich Text Editor data into your Multiple lines of text column without the html tags using your custom form?
Have you enabled the "Use enhanced rich text" option for the Multiple lines of text type column in your SP list?
Based on the needs that you mentioned, I have made a test on my side, and don't have the issue that you mentioned.
Please check if you have enabled the "Use enhanced rich text" option for the Multiple lines of text type column in your SP list.
If you want to save the Rich Text Editor data into your Multiple lines of text type column without the html tags using your custom form, please consider enable the "Use enhanced rich text" option for the Multiple lines of text type column in your SP list as below:
Best regards,