I've been seeing it more and more and they usually have it. Often it's coming from a weaker player. I make the stupid mistake of calling them down too lightly, but I'm starting to think maybe this is a leak of mine by not believing them. So I ask if this line from a player is usually a sign of strength and I should just fold often like a pair or two pair and a straight on a flush board.

Unless there's like a lot of obvious missed draws, yeah this line is overly strong

When a draw hits it's really strong. From a logical standpoint no one is check/calling with air just to wait for a draw to hit so they can bluff it.

When the draws miss, it can sometimes be a desperate bluff. But it can also be a strong hand with scared money waiting for a safe river to get it in.

This is one of the most underbluffed lines in poker. Fish do it for a variety of reasons or lack thereof. When draws hit the reason is obvious. When the board bricks they usually still have it when they lead big. Sometimes they just hit some unimaginable 2-pair. Sometimes they were trapping because they were afraid you'd fold if they raised, but then on the river they decided they were more afraid you'd check back.

In my database river donks are bluffs maybe 10% of the time overall. If you're overdefending against them you're adjusting in the wrong direction.

Edit: Also when playing against fish and weak regs, always assume they have sizing tells and assume their big bets are underbluffed until you see otherwise with a particular player. Fish often like to value bet big and bluff small. Regs sometimes do the opposite but they also often play faceup at low stakes, because value betting big and bluffing small is a decent strategy against a lot of fish.