Why does my admin email address keep changing to something random?

if your admin email changes, I suppose it’s not a good signal, so it’s very important understand why it’s happening.

Some malware usually have this behaviours, so I think you can start from:

• have you checked if your plugins, theme and WP core are updated to latest versions? if no – backup all and proceed with the updates;
• try to check if some of the spam users you mentioned have an adminsitrative role.

Let us know,
cheers.