Blocks reappearing glitch

If the chunk is near the spawn, it's probably spawn protection (See: server.properties) and the player that was interacting with objects in the chunk was a person of a higher rank (in vanilla, this would be an operator).

Determines the radius of the spawn protection. Setting this to 0 will not disable spawn protection. 0 will protect the single block at the spawn point. 1 will protect a 3x3 area centered on the spawn point. 2 will protect 5x5, 3 will protect 7x7, etc. This option is not generated on the first server start and appears when the first player joins. If there are no ops set on the server, the spawn protection will be disabled automatically.

The default protection radius is 16x16x16, which would explain why it is 1 chunk.

I've seen this happen when the area is for certain not spawn protected (for example, mini games where breaking blocks is required). This doesn't explain why it is just one chunk, but it could most certainly be lag. So for anyone where this issue is happening world-wide, not just to one chunk, if at all possible keep breaking the block, I've found that eventually it will break.