First issue, naturally generated cacti have an insanely fast time for cactus fruit regeneration. They regenerate their fruit roughly every 10-15 seconds (I neglected to time it accurately) when the game speed is set all the way down at speed 1.
To observe, simply harvest a cactus plant, and watch it re-fruit in a matter of seconds.
Issue # 2 is related to cacti planted by the player in humus on sand (or planted on humus on sand, un-tilled back to sand).
Planted cacti are not harvest-able, even though they show fruit after maturing. Instead, after a set amount of time, they disappear and reform as freshly planted seedlings again, and in the process of doing so, drop 1-2 fruits on available empty ground spots around them (if any, if all spots are occupied by buildings/other cacti/items on ground, then they don't drop any at all). The maturity time on these do appear to be correct, though, unlike their naturally generated cacti counterparts. I assume this is a glitch, since apple/pear trees planted by the player work as being harvest-able through normal means or the production queue (well, maybe not pear trees after reading the post about those and the auto-harvest production menu glitch)
To observe, right click a planted cacti showing fruits, notice the lack of a harvest option on the right-click menu.
Observe until the maturity timer expires, and observe the cacti devolve back down to a cacti seedling(planted), and drop it's fruit randomly on the surrounding tiles.