This is the expected behavior.
Probably there is just a misunderstanding of nomenclature:
Templates are not meant to paste full messages in.
For this, you would create a message of type HTML Paste.
A "template" is a base framework for messages, not an email message.
The template is meant to be the foundation for multiple email messages and thus should not hold your entire message, only the non-changing elements, like a wrapper.
A template can then be made to hold "slots" for editable content, an explanation can be found here:
How do I include editable Content Boxes in a Content Builder HTML Paste Email?
If you want elements of your message to be editable in layout mode, you would need to:
a) identify the "wrapper", and paste only this into the template.
b) Insert a slot into the template html as described
c) save this as template
d) slice the inner part of your source message (everything not part of the wrapper) into content blocks, which you save as "free forms".
e) Then create a new message "from template".
f) place the free forms inside the slot.
Result: the template still can only be edited in html mode.
Each individual block is editable as free form.