With placement grid on (I tried a few sizes (2, 3, 4, 14) bigger than 1, which works as intended), fast placement off (default tied to restriction on) and with any restriction modes Z 1-6, I can place blocks against the blocks part of the grid and the grid blocks, using offset placement (in one single held right click). I tested the against part with logs and offset placement. Without the offset only the original block and the flower around can be placed.

With the Tie off, only with fast placement the restriction works (it acts like tie is still on), I couldn't even get the flower without fast, only the original block. It's like the Tie is still there.

Conclusion I: The placement grid only properly works with the tie, in fast placement, meaning with a set other restriction (layer, ...) and there is no "None", so no 3D grids.

The expected behavior would be for the grid to have no effect, as it is with the other placement restrictions, while fast is off and the tie on.

Conclusion II: The current working settings (fast placement for grid) are good enough (for now), only the bug in the restriction off state is really problematic, as turning on the grid and not turning on fast placement immediately is naturally expected behavior. A helper message could be appropriate: "Turn on fast block placement and keep placementRestrictionTiedToFast on true."