I had a different, but in some ways similar, experience with the game Elite on the Amstrad CPC. At the time I borrowed it from my friend who raved about it, the elastic band in my tape deck was starting to stretch, and the tape speed was somewhat inconsistent. Listening to it load sounded horrible - you could hear it warbling on the normally steady tones, but generally things loaded just fine so I wasn't massively worried about it.
Anyway, in Elite, you can save and restore your progress, so I did that because I felt like I'd accomplished something. However, after a week or so, I was getting pretty bored that I was just flying from place to place, trading, but not a lot else was happening. I had the occasional fight with another ship on my way to a new planet, but only maybe every 2nd or 3rd flight. It was basically a trading game and nothing much else.
I returned the game to my friend a couple of weeks later and told him how I found it pretty boring. He was surprised and said you get attacked almost every flight. We loaded it up on his CPC, and sure enough, I played for about an hour, and there was lots of combat. Borrowed the game from him again, and this time didn't load up my old save game, and had the same - lots of combat. Reluctantly, I started again, losing all my credits from trading, but suddenly the game was actually fun again.
My best guess is that some data that controlled how much combat action I got had been corrupted in a way that wasn't detected by the checksum, and once that was reloaded it got persisted in every subsequent save. It sounds implausible, but actually most checksum schemes on the CPC don't differentiate between runs of 00 bytes or runs of FF bytes, as they're usually done as mod-255. [0]
[0] checksum code is often a bit like this: IN: A byte that was written, HL previous CRC. ADD A,H: ADC A,0: LD H,A: ADD A,L: ADC A,0: LD L,A [1]
[1] Often called Fletcher-16, it's much simpler on an 8-bit CPU than the pseudo-code on Wikipedia suggests [2] if you pre-initialise the counters to 1 instead of 0
I think Elite on CPC had a Firebird loader, which had its own checksum algorithm different from Amstrad ROM. It had way shorter blocks. It might be weaker against certain patterns more than the stock ROM as you said.