[Gameplay] Garen Q stops him from attacking SR Nexus
Garen's Q (Decisive Strike) makes it so that Garen cannot strike the enemy Nexus for a few seconds. Garen either needs to get rid of the Q buff in order to continue attacking the Nexus, or wait the few seconds. This is a very easy thing to replicate in Summoner's Rift (unsure of other maps). Just start attacking the Nexus with Garen and then activate Q and to see Garen stop attacking. I have played Garen many times to confirm that this bug is here.
This bug has been here for at least a few months (when I started playing as Garen), but I guess it has gone under the radar due to Garen not being a popular pick.