The minecraft:flammable component will not cause custom blocks to be ignited by lava.

Following is the JSON:

"minecraft:flammable": {
  "flame_odds": 5,
  "burn_odds": 20
}

In the video below, I set the Random Tick Speed to 256, and then placed some custom blocks with the above JSON next to the lava. Then I placed another custom blocks with the same one on fire with the flint. The ones placed next to the lava will not be ignited by the lava. Even if I put it in lava.