Just tried it on my Samsung Galaxy S6 with Android 7 and I can confirm that the calendar entries disappear after using MAX POWER SAVING mode from Samsung. But only if I restart the device after the mode is activated. If I switch back from this mode without a reboot only the sync interval is set to manual (which is an already known bug from Samsung and can’t be fixed by us). But at least the calendars are then still there.
@Error101 you need to contact Samsung about it. It seems to be a common issue with their MAX power saving mode - people are affected in different ways by this
Unfortunately, we were never able to reproduce this problem, nor do I have an idea what the cause could be. I assume it’s related to the Samsung firmware (as you can see, both devices are Samsung GT-I91*) which does something different than the default Android Contacts provider. Because Samsung’s firmware isn’t open source, there’s no reliable way to confirm that or find a workaround. The only thing that I can suggest is to use an open-source firmware (AOSP, CyanogenMod etc.)