The bug

When you let a structure block with integrity lower than 1.0 and seed of 0 (= random seed) load a structure every tick, the block at the lowest X, Y and Z coordinate of the structure appears to be less random over time. Compared to the other blocks of the structure, there are longer "streaks" where the block is either loaded or not loaded.

An extreme example (and reproduction steps) can be seen in this video: https://youtu.be/N8azjRO4KM0
Every tick a 8x8x8 cube of blocks is placed and afterwards a structure block loads a 8x8x8 cube with an integrity of 0.5. The block at the lowest X, Y and Z coordinates of the resulting cube stays for a significant amount of time the same.

Code analysis

20w49a, Mojang names

When a random seed is used (value 0) the method net.minecraft.world.level.block.entity.StructureBlockEntity.createRandom(long) creates a new Random instance based on the current milliseconds. Despite the milliseconds value being different every tick, the values are too similar as seed for the JDK Random implementation, as described in Earthcomputer's comment.
A better solution might be to have only one static Random instance which is reused.