I started a new single-player save for 1.6; OS is Windows 10; no error messages or in-game text
I was fishing in the trout derby, and catching lots of high-quality fish (mostly gold and iridium), and when I turned in some golden tags I got deluxe bait, which I put on my fiberglass rod. Ever since then every fish I've caught has been normal quality, no matter how well I catch it. I tried removing the bait, but the issue persisted. I then tried fishing until the bait ran out, and after that it still caught only base-quality fish. I then exited to title and loaded the save again, and the same thing still happens.
I previously fished with normal bait, with no issues; only since I tried the deluxe bait have I experienced this bug.