I understand completely what you are saying here and can get behind the idea of wanting to fix this one particular instance (especially because fixing this feature is pretty easy all things considered)... but at the same time the other way to look at this is from the macro perspective-- that every single other aspect of the game is going to require the DM to "fix" things, so bothering to focus on just this one thing can be thought of as missing the forest through the trees.
I mean, just adding a fifth character to an adventuring party now "underpowers" every single monster in the MM, because they were all designed to be balanced against a party of four. Which means the DM is now going to have to "whip up" a counter-balance to challenge them. The monsters also aren't balanced against the party having magic items (especially quite a few magic items)... which means again as soon as the party starts getting them, the monsters are now "underpowered" and the DM is going to have to work to re-balance encounters and such. Now you combine more than four PCs and they all have magic items... the DM's work is now increased even further. Then you throw in class synergies, optional rules added like feats, any other house rules the table might use... pretty soon there's not a single thing in the game the DM can just use as-is. They are having to eyeball and guesstimate fixes and balances across the entire board all the time throughout the life of the campaign.
When we look at it through that lens... the lens that says that every single thing the PCs have over and above what the baseline was designed against (which I've always believed to be just four characters taken directly from the Basic Rules-- one Fighter, Rogue, Cleric, and Wizard)... the DM is ever and always going to be re-balancing the game all the time for dozens, if not hundreds of different things. Which is fine... it's the job the DM signed up for... but it does mean that just adding in one more thing to account for in the balancing-- dealing with the few extra HP characters get from the Twilight Cleric's ability-- overall is not a big deal at all. The DM just works around it the same way and at the exact same time they are working around everything else.
Long story short (too late!)... fixing a quick and easy rule is fine, and you might as well do it. But its also good to see if any job isn't ultimately necessary to fix if its problems are going to be smoothed over during the normal work of the DM's rebalancing anyway. Because at least then you have less "fixes" to have to keep remembering you've made.
That's not exactly what I meant.
My point was: saying "it's not overpowered because the DM can fix it..." is a meaningless and unhelpful statement as the DM CAN fix anything.
You can't really use THAT as any sort of metric re: power level.