When waterlogged shallow dirt doesn't have a block on one or more sides, but could form an infinite source, it flickers between being waterlogged and not waterlogged as shown in the video.

I should clarify that I'm using the Crucial modpack + optifine and Create
And also that the flickering between states doesn't seem to be only visual

This is a shader issue, yeah, not really much that can be don here.

I thought that didn't sound right, so I removed optifine and tried to replicate again: