Deadeye Plunderer & Reinforcing [Investigating]

Brakkis

When you reinforce this creature, it does not gain any  further +1/+1 per support. Only the first iteration gets this bonus.

//Edited Title -Brigby

Nestor

It will do a reinforcement but not the right way. It adds the number of supports to the base of the creature including the reinforcements. I have had this creature decline in power and toughness from one reinforcement to the next.

Enygma6

This looks like the same bug as Tishana, wherein the card overwrites its own buff instead of adding a new buff to itself when cast again.

Enygma6

Knight of the Reliquary also suffers from this bug.
I was playing a match in Training Grounds, using Knight of the Reliquary and 4 Land Supports in my deck.  
Late in the match I drew 2 copies of the Knight.
By the time I cast the first one, 4 Land cards had ended up in my graveyard.  The Knight is cast at 4/4, and calculates a buff to become 12/12.
A turn later, the Knight has taken damage and is 12/4.
I cast the second Knight, it enters the battlefield and reinforces the existing one to 16/8.  It then flashes as the buff is recalculated, and it remains 16/8.  
There had been no additional Land supports entering my graveyard between the two Knights being cast, so the buff values would be the same.  
  
In this case, as seen with Deadeye Plunder and Tishana, the Knight is overwriting its own buff and re-applying to it's reinforced base values.  It is NOT adding a new buff to the existing Power/Toughness values.

Enygma6

Tested again with the latest Hot Fix.  Tishana is now working properly, but both Deadeye Plunderer and Knight of the Reliquary are still exhibiting this bug.  
Appears to me that this fix might have to be applied on a card-by-card basis.  
I have not gone through all the new Ixalan/Rivals cards to see which give themselves a calculated buff on ETB, but I would not be surprised to find this elsewhere.