When viewing a Lifecycle the Usage tab always says “This lifecycle is not being used in any projects” as per this screenshot. However this is not correct.
Trying to delete the lifecycle shows that it is in use as the validation kicks in and the deletion is prevented.
Feel free to login to our instance if you want to take a look for yourself.
I came across this issue Lifecycles not showing Projects using it in 3.3.12 - #4 by Daniel_Fischer which seems to be exactly the same but was for onprem and has since been resolved.