Moderator's TL;DR:

The way lava damages the player seems to have changed between 1.15.2 and 1.16.1, causing the player to die a little bit faster in 1.16.1 than before.

Original Post

Worth mentioning right on the start, I'm unsure whether this is supposed to be the way it is, but assuming it hasn't been listed on any of the update logs (as far as I've seen), I would assume it's a bug. Lava does far more damage in 1.16 than in 1.15.
I've ran some tests, and with all diamond armours with protection 4 enchants, you could swim in lava for around 28-29 seconds before dying in 1.15, in 1.16 you can only stay in lava with the same armours for around 16-17 seconds. I also came to notice that Netherite armour with protection 4s on all pieces do not grant (at least a noticeable) difference to having diamond armour with the same enchants. Both could only last around 16-17 seconds in lava before dying. I also tested the same but using fire protection armours, and the result is around the same. Full diamond armour with fire protection 4 on all pieces would last only 29-30 seconds in lava before dying, same applies to netherite with the same enchants, meanwhile in 1.15 version you could last up to 50-51 seconds with just diamond armour with fire protection 4s on all pieces. The same damage rate also applied to iron armours. I also tested this without any enchants on the armours, and diamond armours with no enchants could only last for 6-7 seconds before dying, and netherite could go only up to 7-8 seconds. All tests were run without regeneration from food / hunger bar, in vanilla Minecraft.

I think it is pretty self-explanatory, but lava does far more damage in 1.16 than in 1.15, especially biggest difference coming in with good armour designed to withstand the damage of lava. I would assume this would be purposely done, but for the reason that it hasn't been mentioned on the update notes I'm going as far as calling it a bug.