The problem:
When you change the delay of a repeater to 2 or 4 while the input line is on, the repeater will stay on until you turn off the input signal for at least the length of the duration. It will not pick up when the input is turned off shortly, like with a fast clock running into it, unlike when the delay was last changed with the input turned off.
How to replicate:
1. Place a repeater.
2. Attach the repeater to a clock which is on for 2 gameticks, then off for 2 gameticks (1 redstone tick).
3. Change the delay of the repeater to 2 or 4.
4. If this changing is done while the signal was on, the repeater will now be permanently powered.
Method 2 for replication:
Place a repeater.
Set the repeater to 2 or 4 ticks.
Make the input of the repeater a continuous on signal.
Turn this signal into a 2 gametick (1 redstone tick) on, 2 gametick off clock without interupting the signal for longer than 2 gameticks.
The repeater will now stay permenantly on.
Video's:
The video named Correct Behaviour shows the behaviour how it should behave, which happens if you activate the clock from a disabled signal.
The video named Wrong Behaviour shows the bug as replicated using method 2 as described above. Here it can be seen that the repeater remains enabled permenantly.
I'm pretty sure this is working as intended. Repeaters are supposed to lengthen the pulse to the set delay as far as I'm aware. It has been like this since the beginning and is commonly used.
If you want to delay the signal without lengthening the pulse you can place several repeaters on the first delay setting.