At the time of this posting, I did not know a bug thread was started in the engineer forum. I will update this post to try and keep updated information on here. Already submitted an in-game bug report.
A recent fix was applied to the Engineer Toolbelt skill: Mine Field in the recent (5/28) patch. Once the mine field is set down, there should be a sequential skill to detonate the mine field. The recent fix now makes it so that the sequential skill disappears when an allied player walks over the mines from the mine field. To top it off, the mine field goes on a 20s cooldown, ignoring any toolbelt recharge rate increase from the traitline.
Only time the recharge ends is if nobody steps on a mine for the 20s cooldown. The Mine Field skill is then available for use again, but stepping on the old mines before laying down the new Mine Field would send the skill back into a 20s cooldown.
Entities that seem to trigger the skill cooldown includes: allied NPCs, allied players, enemies, and the engineer himself.
Makes no difference whether traited for Speedy Gadgets, Rifled Barrels, or not. Possibly not trait related.
Surprisingly, Throw Mine, the main skill, works fine as far as I can tell.
Tested in WvW and Lion’s Arch.
Other information from engineer forum thread: Seems to be affect underwater combat as well.