I can explain why this happens. When Platform behavior is enabled and you control the character only with Platform actions, it prevents it from falling through solids and always keeps it about 0.5px above the floor.
When you move the character with actions like "Set Y", on some ticks it may move a little further and actually overlap the floor. (even a fraction of a pixel is still considered overlapping)
"Is on floor" condition moves the character 1px down to check if it will overlap a solid floor. However, if it's already stuck in solid, this condition returns false. That's why it doesn't trigger and your character continues falling down.
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If you need to disable the Platform behavior for some reason, you shouldn't use its conditions to check for floors and walls, use "Sprite Is overlapping" and "Sprite On collisions" instead.