Yes, nearly 3 years later and explanation of masteries is still not in-game.  What harm would it cause to have a transparent explanation of the effects of masteries on the page where you actually select the masteries?

I have read that people have tested it, and it DOES work on continuous heal buffs.

https://www.reddit.com/r/RaidShadowLegends/comments/lir19y/healing_masteries_do_work_with_continuous_heals_i/

I just reworked three Continuous Heal champs. It made a difference in durability for sure. My Black Knight heals to full if he's not killed in one turn as someone else mentioned in the link above. He didn't before.

I've found if your using that mastery it will help to pair any champions with the ones you give the healing help from defense n down further there's mastery that speed up how many turns you gotta wait so me personally if I use the add 5 percent to all heals cast pair w champions I gave the heals boost from defense and further down speed up the turn meter so they can do it more often