Buffs not Applied after P/T Changes [Investigating]

Some creatures that allow for the changing of their power and toughness are not behaving as I think they should. Two creatures that come to mind are Protean Thaumaturge and Glasspool Mimic, but there may be others. These creatures have a feature that allows the player to change the creature's P/T to match another creature in play. However, if there is a support in play that buffs Protean or Glasspool it is not applied in addition to the new P/T. 

For example, if I have Glasspool in first position with Shadowspear on the board, my Glasspool is 4/4 (1/1 plus +3/+3). My opponent has an 8/8  critter out. On my next turn I cast another Glasspool and select to match the P/T of my opponent's 8/8 creature. My Glasspool should get 8/8 then +3/+3, but the buff is not applied. However, if the Shadowspear is destroyed, Glasspool gets bumped down to 5/5. 

If this is behaving "as designed," then it is poor design and should be fixed.