Oh yea you should be very concerned. Probably jerry rigged to sale house.

What does it look like when it is wet? Are you seeing the "grout" absorbing water and changing color?

Doesn’t look like it’s absorbing water. It looks a little darker while taking a shower but not much.

@Dewayne are resin based grouts softer than regular grout?

No, polyurethane, acrylic, or epoxy grouts are not softer than regular grout, when they set and cures properly. Until it sets, it can resemble playdough though. But it isn't designed to work with the big giant spaces between pebbles or penny tiles. It works great on any other type of installation. TEC's polyurethane based PowerGrout has been notorious for not setting up, and needing a hardener. But it too should never be used with pebbles.