In this project, the BitBtns have the setting NumGlyphs=2. This means that the icons contain both a normal and a disabled image. The default icons loaded by Lazarus, however, contain only the normal image - the disabled image is derived from the normal image by image manipulation. So, when you switch NumGlyphs to 1 the full image will be displayed again.
I checked Laz 2.0.12. When I add a TBitBtn here and set its Kind to bkOK the icon is displayed correctly, and NumGlyphs is 1. Whatever value I set for NumGlyphs, the image does not change. However, when I assign a 2-glyph image the image is correctly split and displayed after setting NumGlyphs to 2.
I also checked Delphi XE 10.3. Now, when I add a TBitBtn and set Kind to bkOK, NumGlyph jumps to 2, and when I return it to 1 I see a disabled image next to the normal image. This is different from Lazarus which uses single glyphs.
Therefore, i think that your form was imported from Delphi, and the NumGlyphs settings has been kept after the import.