Shrinking Borders: At night time, the world border will shrink to the size of two blocks wide. Anything outside the world border is instantly deleted and destroyed! So don’t stay outside at dusk for too long... Astral Companionship: ...
So to record an area of visible chunks, you must add one chunk to your border, e.g. recording a visible area from -5, -5 to 5, 5 you must record between -6, -6 and 6, 6. To record an area of chunks on your server you can run /replay start chunks from <chunkFromX> <...