I have had this problem for the last 7-10 days on my level 111 druid and switched to leveling a priest alt. As soon as I hit 110 on him started getting the same problem. I tried putting in a ticket as soon as it started on the druid and have gotten some help (every 30 hours I will get a response). I then tried adding to existing ticket for a callback response and it constantly tells me there is a problem on your end and to try again later. I don’t want to add to the existing ticket as I am REALLY tired of waiting 30 hours for a response, they ask a question, I answer and then another 30 hours of waiting. I just want some help figuring this out. I am trying to take advantage of the 15% boost from the anniversary as well as all my heirloom gear to get all my alts to 120 and this issue will make one character take months to get to 120 (I am getting 20 XP for mobs one level below me on my druid). As it stands the frustration level just increases every day.
Where are you currently leveling? Once you hit 110, your XP for quests etc. in Legion drops dramatically. You have to move on to the Battle for Azeroth content to continue getting full XP.
Edit: As a side note, Live Chat is for Billing or Tech Support issues, so it really shouldn’t be used for something like this 
Perl is correct. It looks like your priest and your druid are still currently in Legion zones. You’ve reached the level cap in those zones and XP gains will be less there. Once you move into Battle for Azeroth content, you’ll see the XP gains return to normal.
As others have said…
Once you reach the top level for an expansion, XP gains in that expansion’s content drop by 90%.
If you are 110 and still doing Legion content, you will see this.
I have taken the druid to the new area in BFA…somewhere around silithus if I remember right. Killed a few mobs in that location at same level as me and still got about 150xp per mob kill. The mobs I am killing in the legion zones are only one level below my priest and still getting very low xp. Before 110 I was getting 1888xp for a mob kill and after 110 it is 160xp. I understand it should drop but it shouldn’t drop that drastically I would assume. I took some screenshots to show the change but got no idea how to post them in here.
If it helps I am leveling with an almost complete set of heirloom gear as well as the 15% anniversary buff and only leveling while using rested xp for the doubled xp for that as well.
I’m not 100% on this, but I don’t believe rested XP stacks with heirlooms bonuses.
This is spot on. As stated above, once you hit 110, Legion zones XP drops by 90%. It’s to encourage you to move to the proper zone for the level. (Note that all of this will be obsolete with the new expansion.)
10% of 1800 is 180. So the XP gains are right where they are intended.
Around Silithus? Are you actually killing the azerite elementals in the new Silithus (with the huge sword)?
I’ve never killed any of them, so I do not know what the base is for their XP. But it’s possible that Silithus is still considered a level 60 zone, and has the 90% nerf. If this is true, I’m not sure if that’s a bug or not.
I would advise to keep going with that opening questline, get to Zandalar, and see what the mobs in those three zones give you.
It does stack.
They are scaled but I do not believe it is considered current content there.
Move on through the opening quest lines and get to Zandalari or Kul Tiras depending on faction. At that point you should see vast improvement.
Silithus is Legion content, not BfA. The only BfA content there is the Heart Chamber and the Champions of Azeroth reputation vendor.
Edit:
The mobs in Silithus with the sword were quest mobs leading up to draining the Artifact weapons at the end of the expansion.
BFA areas are Kul Tiras and Zandalar. Gotta go there. Silithus doesn’t count.
(Unrelated side-note: it’s not good when you miss the ‘o’ in ‘count’, thank goodness I caught it before posting. )
As an aside to this, I think it would have prevented the post for a bad word.
Don’t ask how I know this with my history of typing on my phone though. 
Incidentally, this is another common question that will be going away with time; Shadowlands to be exact. Level revamp and all. Ninja loot questions vanished (then came back cause of Classic, but still not as prevalent), PvP server complaints have diminished, other things too I’m sure I’m forgetting atm.
Just to add, if your after fast levelling during the anniversary event, go hit AV, its like 3 hours to get from 110-120