When you hook up 2 redstone components and power one directly with redstone dust, and the other with a repeater set to a 1-tick delay, Both will be activated at the same time, as if the repeater has no delay. This leads to non-deterministic outcomes, wherein the component that will be powered first is pretty much random. See video and pictures for example:

https://youtu.be/z5BXgd2fbP8

Note that at the time of recording, I thought the bug was with pistons, and didn't realize it also applied to redstone lamps and other blocks, too. I now believe this bug is either caused by repeaters/comparators, by redstone dust, or by just incorrect redstone timings in general. I've also noticed that repeaters, comparators, redstone torches, and redstone dust are powered with the correct timing delay when repeaters are powering them, either directly or with redstone dust in-between, but the affected components listed below will still be powered as though there were no dust between them and the repeater (meaning that you can power a line with dust -> repeater -> 2nd dust -> redstone lamp, and the redstone lamp will come on at the same time as the 1st dust and repeater, and actually 1 redstone tick before the 2nd dust that's supposed to be powering it! This bug almost certainly has something to do with the way that redstone components are powered in MCPE ignoring certain factors (this bug is another example: MCPE-11871).

List of blocks this bug affects:

• pistons
• sticky pistons
• dispensers
• droppers
• redstone lamps
• observers (placing redstone dust on the input of one and a repeater on the other and activating both simultaneously will cause both observers to be activated at the same time)
• note blocks

List of blocks this bug doesn't seem to affect:

• repeaters (the proper delay will be applied to repeaters being powered by other repeaters or redstone dust coming from other repeaters)
• comparators (same as with repeaters)
• redstone torches (by sending the dust/repeaters into blocks to invert the torches)
• redstone dust (but components hooked up like repeater -> dust -> piston will actually be powered 1 redstone tick before the dust and in the same tick as the repeater is powered)

This bug does not occur on the Java edition, as repeaters set to a 1-tick delay always add a 1 redstone tick delay no matter what they're hooked up to. I'm not sure how long this bug has been in the game, but it has been there since at least 0.15.3. This bug causes various redstone contraptions to act as if they were being affected by MCPE-16371, since there's no distinction between a repeater powering something and redstone dust powering something in many cases, causing several things to be powered at the same time, which results in random/unpredictable behavior.