iserith
Magic Wordsmith
Speaking as another DM on the same page as @jgsugden, I think that's fine. However, if every monster always automatically skipped past him to attack softer targets, that would be different.
Again, speaking only for myself, I try to choose targets for my monsters that they would choose. I use, or try to use, the targeting methodology that they would use. So, for instance, a kobold is probably going to target the easiest looking enemy. An orc warlord will probably engage the toughest looking warrior, but might lose interest after a couple of rounds of trading zero damage. An ooze will probably attack the closest creature; a wolf is likely to strike at an enemy with its pack mates surrounding it, and a zombie will probably try to hit whatever hit it last.
Sometimes there's more to drawing fire than just standing there dodging. The cleric in the OP is more likely to draw fire if he moves into the enemies' formation, if he shows that he's a threat instead of just dodging, if he uses an action to insult his foes. There are lots of ways, at least in my game and my experience, to increase the odds that you'll be the target of choice of the baddies.
Occasionally? Or as a consistent thing? Frequency matters.
Occasionally: no problem. Some monsters might be fully aware they can't reliably beat heavy armor with their weapons. Or they might go after other PCs for other reasons. For example, animated dead might mob the rogue who opened a small chest in a crypt, thereby defiling their place of rest.
Consistently: I'd see that as a problem.
I figured this would be a likely (and reasonable) response. My take on that is that I don't know what a monster would do. I only know what they could do, which is whatever I say they do, for whatever reason I say they do it. And I can come up with all manner of acceptable fictional reasons on the fly why a monster would avoid the heavily-armored PC to attack someone else. If I can, anyone can. So knowing this, as a player, I don't think I could become annoyed that the DM was specifically not targeting me. After all:
He's not taking any damage, is he? He's free to cast Bless with no worry of his concentration being interrupted, for instance....
Right. My investment in heavy armor and the feat means that I have effectively deterred monsters from attacking me which is in some ways better than having the monsters swing, miss most of the time, but hit me sometimes. (If the party lacks any other tanks, it's less good. Assuming I'm a team player.) Freed from that threat, I don't have to worry about losing Concentration and probably don't have to take War Caster.
I don't really feel it's the role of the DM to provide some kind of payoff for a player's investment as if there's an obligation to have all these monsters attack a character and miss because his or her player took heavy armor and a feat. Rather, I think it's on the player to make that investment pay off. In my Planescape game, given half a chance, I will target the wizard or a squishy NPC sidekick (or a barbarian in a pinch). The paladin and the fighter are armored up. So they take it upon themselves to position themselves better, using grappling, shoving, dragging, spells, etc. to arrange the combat in such a way to make themselves the only viable targets. They goad, talk smack, and use the terrain to keep themselves in the line of fire. I think it makes for better scenes when they have to work for it and a better payoff when they achieve victory.
That's just me though.