weird insert of backslashes in strings

[axel]

Configuration Information [Automatically generated, do not change]:
Machine: i686
OS: linux-gnu
Compiler: gcc
Compilation CFLAGS: -fstack-protector -Wno-parentheses -Wno-format-security
uname output: Linux bongo 2.6.32-042stab134.8 #1 SMP Fri Dec 7 17:16:09 MSK 
2018 i686 i686 i386 GNU/Linux
Machine Type: i686-pc-linux-gnu

Bash Version: 5.0
Patch Level: 0
Release Status: release

Description:
This script "weirdbackslashes":

$ cat weirdbackslashes
#===begin
PRE='\/'
echo 'define'${PRE}'\
/'
#===end

produces with bash4:
$ bash weirdbackslashes
define\/\
/

and with bash5:
$ bash5 weirdbackslashes
\d\e\f\i\n\e\/\\\
/

Repeat-By:
see Description