The bug

When a weighted pressure plate initially detects an entity on top of it, it will always consistently output a power level of 1 after two game ticks. However, that is not the case if an additional mob is added that should increase the power level. In this case, there will always be a longer delay of a seemingly random amount of time.

Both light and heavy weighted pressure plates are affected. For heavy weighted pressure plates, the delay seems to be significantly longer than for light weighted pressure plates.

I discovered this attempting to use the weighted pressure plate for its intended use of detecting the number of entities in a given area, specifically I was trying to detect whether an item stream had dissimilar items in it because different items types would not stack and produce a signal strength greater than 1. However due to the random delay for consecutive items, it would often pass over the plate before it was detected making the setup effectively useless.

To reproduce

1. Place a weighted pressure plate and connect a line of redstone dust to it
2. Summon a mob on top of the pressure plate
   → Observe that the pressure plate outputs a redstone signal with a strength of 1 exactly two game ticks after the mob has been placed on the pressure plate
3. Summon another mob
   → The delay is longer than two game ticks
4. Repeat
   → Observe that the delay is not the same every time

Demo

Here's a test world showcasing this issue: MC-179708.zip
In this video you can see that the delays for level increases are inconsistent: https://youtu.be/IEJARc78FyE