Removal of fire resistance when splashing it in lava

• In the nether
• Fell into lava
• Splashed fire res.
• Gave effect and then removed it
• Got the achievement 'Stayin' Frosty' (Swim in lava while having the fire resistance effect)
• which is proof of the effect being given and then removed 

I don't know whether this is because I used a splash potion but I just found it strange and i lost all of my stuff.

How to Reproduce:

1. Go to the Nether
2. Jump into lava ocean
3. Splash Fire Resistance while falling
4. If it doesn't work, retry and if it isn't happening then I am sorry for wasting your time