I have two max level Treasure Hunters and since yesterday, Tuesday May 13, I have seen only one grade 6 gemstone mission available. I sent out five companions for each in grade 6 missions to try and get a gemstone mission to appear six times by now since yesterday and still have only gotten one gemstone mission. Is this a stealth change?

Grade 6 missions (across ALL gathering and mission crew skills) have a far more limited pool of missions; limited to moderate and abundant missions only. This was the materials limiting strategy employed back when grade 6 was the top end. If you are looking for bountiful and rich grade 6 missions you will not find any that generate materials. Rich grade 6 missions are strictly limited to grade 5 companion gifts. This has been the way it is since launch.