I’ve seen this a few times now. If one enters a value into the Display Name field for an action, but then later deletes it, the actual displayed name still remains as the old entered value, instead of defaulting to the action name.
This bug is of similar nature to a number of different small bugs I’ve seen as well, like action names within workflows not sticking around upon initial saves. Perhaps they have similar causes.
Steps to reproduce, with respective images:
1. create new action, save
2. enter a value into Display Name field, save
3. delete value from Display Name field, save