So something is wrong here.
When i write file name with extension, which not exists. It opens file under the cursor, not new one i have just created.
I am using last beta version.
Just tested in Double Commander Version: 1.1.30 gamma Revision: 815 System: Windows 11 25H2 x86_64 and it works as expected.

opens file under the cursor

maybe you just put %p0 in the parameters field of tools>editor or the edit action in file associations?

Yes, thats it!
I was copying params from file association to params for external editor and didn't delete %p.