While blocks like sand and gravel fall as soon as they're placed, blocks created with MCreator won't fall until "tick rate" amount of ticks have passed. This means that a block with tick rate 10 will fall after 0.5 seconds, a block with tick rate 100 will fall after 5 seconds and so on. In addition to that, the block won't fall at all if the "On tick update" event triggers a procedure.
In the example workspace, red block triggers a procedure on tick update and doesn't fall, green block has a tick rate of 10 and falls after 0.5 seconds, blue block has a tick rate of 100 and falls after 5 seconds.
This is normal. MCreator uses Minecraft vanilla gravity system which checks for the block below each tick.
Regarding custom update tick trigger overriding gravity, this is indeed a bug. We will fix this bug in next updates.