We have been discussing that in this thread:
https://community.puzzlequest3.com/t/investigating-dwarves-boots-can-only-trigger-once-per-combat/
To confirm, the bonus is applied for making 5 individual matches or more (eg. 3 green, 4 yellow, 3 red, 5 blue & 3 purple) and not a match of 5 gems or more in a single match.
If you can grab a video/screen recording of this occurring and submit it in a ticket - that would be greatly helpful!