Experimental branch, TheRunningManZ community server, 20:09 06/02/2019. A few min after https://feedback.bistudio.com/T136806 basically. Have not had this happen since.
After helping my friend and clearing a horde of infected from the front of the house he was in with my gun and knife I got aggroed by one and couldn't hit it with my knife, I ended up shooting it. After that more infected came running and I tried shooing them (1:45) but my shots were going through them and hitting behind them (1:55) while on others they were hitting fine.