-support
I truly don't think an entire rule for that is necessary. If a discord/teamspeak call takes place to discuss evidence or whatever it may be, the staff member should know by default that they need to hear both sides when they return in-game anyway. Instead of adding an entire rule, maybe this should be emphasized by a higher ranking staff member of the staff team so that whoever has already made the mistake of going straight to punishment, can change their ways.
I had this same situation yesterday. I answered a report and before bringing whoever was reported, I had the victim show me the evidence first. The evidence showed that the reported player did in fact break a rule, but I still ended up bringing him to the sit in order to hear his side. No way would I have gone straight to warning them.