When "deleting time" in time report its not registered by gitpab
When using the time tracking report on issue, and delete hours which was added earlier, its not registered in the gitpab overview

It seems that it is not a very good ideology when someone can delete spent time marked by the developer. If need to remove spent time, a developer must mark time with the sub-zero value, for example /spent -4h. This approach is safer for developers.
I totally agree with you, but it seems that I cannot "disable" this option. And as I cannot disable it, I am not able to control if anyone use this feature.
Currently, there are no options to delete spent time as you want. Only one way - delete the database and load all history again.
this is fine, if one-day gitpab supports those changes, it would be great :). else I will survive without :)