I have tested two seperate mods, although eerily similar in design, at least on a world with cheats enabled so not many advancements to be viewed. I tested Better Advancements (0.4.2.16) by way2muchnoise, and Paginated Advancements & Custom Frames (2.4.2) by DaFuqs. That is their names on both modrinth and github.
This only occurs when pressing the advancement hotkey, which is L by default. The blur will briefly flicker on closing of the menu.

Things that stay the same:
Version: 1.21
Fabric Loader Version: 0.15.11
Mods installed: Fabric API (0.100.6+1.21), Mod Menu (11.0.1), YACL (3.5.0+1.21-fabric), Blur+ (4.1.0)

To recreate:
Download either of the mods, and then hit the hotkey that opens the advancements (default is L)

I will attach two screenshots below:

Better Advancements

Paginated Advancements & Custom Frames