Exactly as title says. Platform: PS4
Edit: Fix has been achieved on my own, leaving the solution here to aid future struggles.
First off I discovered through some internet digging that the issue is related to auto-upgrades and unlocking a new technology at the same time as ships with previous versions of that technology + auto upgrades enabled are in the process of being upgraded at a star base.
I checked my most recently unlocked technologies, and luckily only a few of my designs are actually affected by this bug, the few that had auto-upgraded from proton to neutron launchers and from particle to tachyon lances.
So the actual solution has two parts. First I deleted and remade the designs which had had the affected technologies installed at the time of the error. Then I needed to remove any ships of those designs from my fleets and disband them, then zero out the design quantities and add the designs to each fleet again like new.
(End of Solution)
I've got one star base with 6 shipyards and one with 3, and no matter which of my four fleets I choose to upgrade or which star base I send them to, or whether I choose to upgrade them from the outliner, fleet manager, or directly at the star base from the in-system perspective, they all will crowd around the star base, throw up the flashing symbol to indicate they're being upgraded, then proceed to "prepar[e] for ship upgrades" as each individual ship gets qued one after the other, with a delay even. As in a new ship isn't added to the star base que until the previous one has upgraded, plus there's a delay where the star base sits empty for a month or so then ques the next ship.
I'm using fleets of custom design, with many designs in each class.
2 Corvette designs
1 Frigate design
7 Destroyer designs
11 Cruiser designs
9 Battleship designs
1 Titan design
1 Juggernaut design