How to reproduce:
1. Open the attached “IN-131705_Tilemap_brush” project
2. Open the “scene” Scene
3. In the Tile Palette window select the “Triangle” sprite
4. From the brush menu at the top of the Tile Palette window, select the “Paint a filled box with active brush” option
5. In the Scene paint a 3 by 3 area with the brush
6. Observe the Scene view

Actual result: Only one triangle gets painted in the selected area; to unhide the other triangles, you need to hover over the selected area with your mouse pointer
Expected result: The selected area is filled with triangles

Reproducible with: 6000.4.0a2(79bd4f638ac0), 6000.4.0b5, 6000.5.0a5
Not reproducible with: 6000.0.66f1, 6000.3.6f1(732c1ec89f18), 6000.4.0a1(b27b8eafe426)

Reproducible on: Windows 11
Not reproducible on: No other environments tested

Note: Certain selection sizes work as intended: 1 by 1, 1 by 2, 1 by 3, 1 by 4 and 2 by 2