When I container-grow mint, and catnip in particular, I get large bushy plants that eventually just die, for no obvious reason. I suspected that since mint loves very moist soil, root rot was an issue. These days, I occasionally treat their soil with dilute hydrogen peroxide (two parts water, one part 3% peroxide), and root rot seems to be a thing of the past.

That is a useful tip. I would agree that mints in this area needs the moisture. I haven't dealt with root rot but will keep this in mind.

Spearmint often grows in shallow water here. In fact most of the mints I've grown were water lovers. Not sure what mint you had that had moisture/root issues.

Mint needs to walk, it uses all its nutrients within 2 years and will pretty much die off. I have had 3 different mints I grow, the mojito mint died off this past summer in the drouth. The peppermint is in a raised bed and allowed to travel til I yank it up. Have even dumped a bag of new dirt on it and it returns from underneath, is at least 7 years old. This last 19* cold spell bit it back to ground level, but it popped back up within 2 weeks, so has the Kentucky Colonel Spearmint, going on 5 years.

Mint grows like weed all by itself at several spots in my garden. It doesn't require much water at all here in Istanbul, Turkiye with Mediterranean climate where summers tend to be on the dry side. Mint loves to be pruned in the spring and early summer. So I pick often and enjoy some mint tea with lemon and honey once or twice a week. It's delicious hot or cold.

Also, every winter it "dies off" completely and then come spring regrows from the roots left underground.

Your mint is very well behaved, chuckl, I still have my catnip in its black pot, buried near my tabasco plant in a raised trough, hard to keep water to it, so it is also protected by a wood plank covering the top of the plant ( protects from direct sun, animals, etc,) has plenty of drainage, but I need to water thoroughly every other day.
I would guess that the pot of mint @DougL has, needs more root room - but every plant will die if stagnant in one place when its nutrient specific green leaves run out of what they need.