The title is self-explanatory- I was placing and breaking Glowstone, and the light from it seemed to flicker quickly at chunk borders each time I placed/broke it. You might need a high refresh rate to be able to see it, because it is quite fast- I doubt anyone playing with 60FPS or lower is going to see this. (If it helps, my refresh rate is about 165, and I was using Sodium to get better performance in-game.)
Version is 1.18.

I've tried again with no mods aside from Starlight, apparently this only happens with Chunk Builder switched to Semi-Blocking.
I tried in vanilla and got the same result, which is odd, as I've never seen this before, and I've played it unmodded for a while.
The issue is with vanilla, so I'll close this issue now. Thank you for the help!

try without sodium

Are you able to reproduce this without Sodium?

My best guess is that this is from the threaded chunk updates in 1.18.

Make sure Always Defer Chunk Updates isn't checked in Sodium, and if the issue exists within Vanilla too, try setting Chunk Builder to something other than Threaded. Fully Blocking would be the behavior of 1.17 and earlier.