Describe the bug
does the following every ~1 seconds due to it calculating over a distance:
autosave-interval-seconds: 300
Expected behavior
does it every 300 seconds.
Versions
1.21.1
ServerCore:
1.5.5+1.21.1
Minecraft:
using fabric
Mod incompatibilities
No other mods present.

I'm not entirely sure what you meant. Does it run auto saves every 1 second when auto-save-interval: 300?
The actual calculated time until the next autosave is dependent on the set tickrate of the server (from /tick), is that what you meant by 'a distance'?

I also seem unable to reproduce this if the above is the issue.

it saves every 1 second with auto-save-interval: 300(it appears to have been caused by testing mipmap.)

All it really does is replace a single constant value of 300 in vanilla for computing the next gametick the game will autosave at. Keep in mind that there are other ways to trigger saves, such as /save-all or opening the escape menu.

If you are getting this issue with a value of 300 with servercore you should be able to pull it off without it aswell. In any case I don't seem able to reproduce it, please make sure the saves aren't actually caused by any of the above reasons.