Ummmm, the OP was talking about FISHING dudes. Not general inventory woes. RTP. And the complaint is ligit. Every fish has optional sizes, every size has to have its own slot. Fish don't like the bait you brought so need at least two baits. Your slots are filled way too soon if in fact when you get back to the fishing spot everything hasn't despawned.

On my recent start before I found the trader and got a pole and simple bait there were fish everywhere, all the time, jumping and cavorting, sometimes throwing themselves at my feet. Got pole and bait and how fish are at best sporadic. On the first 10 spots I had marked early on, 2 had fish and after I caught one fish at a spot, the rest disappeared. It was so frustrating. Am at level 16 at fishing but can't really bother with it anymore. Find a spot, set down portal, get rod and bait, return, no fish. Go back, dump stuff and continue on, rinse and repeat. Carry rod and bait, down to maybe 8 open spots total. Grrrrrrr.
Go to swamp, one full day of fishing and finding fish spots. One lousy herring. Two coral fish that I got by jumping in the water and pushing them to shore... and a puffer fish that jumped in my arms, total waste of time.

Or on those rare times where fish are still there all I get are that fish wont take the bait, or my line breaks, or by then my food is old and tired so can't bring the fish in. Fishing used to at least be relaxing and moderately reliable, but not any more.

Five abominations I killed yesterday, not a single trophy. Used to be it was silly how many trophies they dropped, not any more. I just don't understand why it has to be so painfully annoying.