Previously (at least in 9.6) previous_buf hasn’t been used for any kinds of \e subcommand. Now if query_buf is empty previous buffer contents is used.
IIUC the complaint is that it is no longer possible to use \edit to generate a completely empty temporary query that can then be written from scratch.
The v10 behavior is desirable so that leaves either learning a new idiom to accomplish your goal or adding something like "\edit -" to explicitly invoke the desired behavior.
Doing:
# SELECT <enter>
# \edit
Seems reasonably straight forward to get a editor for a newly query instead of the previous one.
