Observers detect an update if a redstone-wire gets powered and unpowered in the same tick

if a redstone wire (or hopper/powered_rail/activator_rail)
gets powered and unpowered in the same tick (easy to reproduce when connecting both sides of a repeater clock to the same hopper/powered_rail/...)
an observer detects an update altough nothing visibly changes

only happens with redstone components that don't have an delay

Edit: added screenshot; you can also replace the redstone dust in front of the observer with a powered rail or hopper