Visibility under lava does not increase with higher level of Fire Resistance. Very likely it is a feature request, however, it seems illogical that higher levels do not increase the visibility. Now it would be use for the level possibility for the /effect command, even if you use the fire resistance.
Use the following command:
/effect give @p minecraft:fire_resistance 100 100/effect give @p minecraft:fire_resistance 100 100
Thank you for your report!
However, we are already tracking this issue, and this report is a Duplicate of MC-188499, which has been marked as Works as Intended - this is an intentional game feature. If you have a different opinion, please visit the Feedback website.
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