Manual summary breaks Markdown formatting by inserting extra newlines. This causes the Summary and rest of content to be placed in separate <p> elements even if they should not have been. This treats the <!--more--> separator differently from any other <!-- --> comment and does not appear to be documented.

What version of Hugo are you using (hugo version)?

$ hugo version
hugo v0.118.2-da7983ac4b94d97d776d7c2405040de97e95c03d+extended darwin/amd64 BuildDate=2023-08-31T11:23:51Z VendorInfo=brew

Does this issue reproduce with the latest release?

Yes