AGPL is a “weak” copyleft license, not strong. It basically means not all derivatives need to be open sourced. If you modify AGPL code but never distribute it or provide it as a service over a network to users, you don’t have to provide source.
No free or open source license requires private modifications to be shared. Doing so would violate your privacy, and goes against the 'you can use this software for any purpose' requirement.
> or [never] provide it as a service over a network to users, you don’t have to provide source.
Hell, AGPL is one of the few licenses that does require you to share modifications, if you provide it as a network service. Again: strong copyleft.