I actually don’t know what causes this so I’ll have to investigate, sorry about the delay but I promise this will get fixed.I made a post November 2018, but it seems the issue hasn´t been adressed. So in case you overlooked it:
Certain Blank Sets take over the HP from the Pokemon you looked up earlier. Example: If I open the calc, it shows me Abomasnow with 321 HP. Now I search for Muk (Blank Set) and Muk appears with 321 HP (at full HP). This is also true for Altaria (and I assume other Pokemon too).