chan_sip is lame duck code. It’s behaviour won’t have changed in years, and it used to support bare IP addresses. I can’t think why it should try to append a network mask length field.
The only significant code between extracting the parameter of host= and calling getaddrinfo is asterisk/netsock2.c at 18.9 · asterisk/asterisk · GitHub and that doesn’t add / anything. It looks like you will see before and after logged if you set debug to, at least, level 5.
Did you manipulate the sip.conf file directly, or did you put the value into a GUI form. In the latter case, what actually ended up in the configuration file?