It seems to only happen on that specific block. Heres comparison between OptiFine and Continuity (using Fabulously Optimized)
OptiFine:

Continuity (Fabulously Optimized):

The resource pack I am using is JustTimm's Vanilla Additions.

Is the block in the same place? If so, there is no guarantee that the random texture at a specific location with Continuity will be exactly the same as with Optifine because Continuity does not calculate which random texture to use exactly like Optifine. Try placing some more blocks.

Is the block in the same place? If so, there is no guarantee that the random texture at a specific location with Continuity will be exactly the same as with Optifine because Continuity does not calculate which random texture to use exactly like Optifine. Try placing some more blocks.

yes, it is in the exact same place. it is like this for all flowering azaleas placed in the world. heres more comparison images:
optifine:

continuity (fabulously optimized):

I tested this and the issue is actually that any sort of overlay does not render on azalea bushes. I am not sure why or how to fix it yet, but I will investigate further later. I suspect that the overlay quad is being rendered under the azalea quad and thus never appears.