Heritability is a pretty funky concept because it's contextual to a certain point in time, environment, and population, effectively.

An example I like is that if you measured the heritability of depression in 2015, and then you measured the heritability of depression in 2021, you would likely see changes due to environmental effects (namely, there's the pandemic/lockdowns and this could conceivably cause more people to experience depressive symptoms). Let's assume we make those measurements and the rate of depression did increase, and we could tie it causally to the pandemic or related events.

In that scenario, the heritability of depression would have decreased. I don't think anyone would argue there were massive genetic changes in that 6 year time period on a population scale, but the environment changed in a way that affected the population as a whole, so the proportion of the effect on the trait which is genetically explained decreased.

For something like lifespan in the above example, you can imagine that in a period of wartime, famine, or widespread disease the heritability would also decrease in many scenarios (if random chance is ending a lot of lives early, how long the tail of lifespan is influenced genetically is much less important).

Given that note, it's generally tricky to talk about whether heritability increases or decreases, but with more accurate estimates of how genetic predispositions form you could see the heritability of certain traits increase with the environment held stable, as there's certainly ones that may be underestimated or genetic factors that aren't currently accounted for in many traits.

*edit: I realized I never mentioned the other thing I wanted to mention writing this! since you mentioned what the percent heritability means here, I think the best way to think of it is just "the proportion of phenotypic variation for this trait in a measured population which is explained by genetic variation." So it's dependent on the amount of variation in several aspects (environmental, genetic, phenotypic).

Some epigenetic effects are semi-heritable too, eg maternal exposure can be transmitted. That's in addition to environmental effects like you mentioned. Two otherwise identical cohorts can inherit the same genetic predisposition for depression where one manifests and another does not entirely due to their circumstances.

Evolution is just super super messy.