Ding, ding, ding, ding.
This is a light bulb moment. I now believe that the problem I had reported here, nearly 10 months ago, was the beginning of my current infection problem with the 2 Coopers FVs. The second and third batch discussed here, which were OK, I must have managed to get out and bottle before it soured. This is when it started, and I've been very lucky to only have to dump a few batches.
I think I recall the white powdery surface on these early problem brews, but I guess I didn't relate it. Even now, with the problem well known to me, if I take the beer out early enough and bottle it before it sours, it still drinks OK. This was probably the difference between the 2 batches discussed in this thread. I managed to save the second and third batches by luck.
Edit: Answering my own questions.
1) What about the 4 brews (2 double batches) that were OK in this thread? There are 2 likely answers here.
a. I took them out of the FV's early enough that they hadn't yet soured. The first "bad" batch was 10 days in the FV. Even the 3rd batch here, which was questionable, had some of the same characteristics of this infection. I mentioned over-priming or gassy bottles. 10 days is too long for this infected beer to stay in the FVs. This is still happening. I just got lucky.
b. Winter. The timing of this is colder weather, which probably gave me a bit of leniency in the severity of the infection.
2) Why didn't I connect these occurrences as the same issue?
a. After the second and third batches seemed OK, I just assumed I'd cocked something up with the first one that was off.
b. I hadn't at that point realised it was common to these two FVs, although lucky for me, I did write about them here.
c. Over the period immediately after I posted this, my brewing took a major downturn. Firstly, I had Covid and was really crook for 2 weeks followed by a trip to Qld, so no brewing at all. When I came back, I was busy helping my 3 kids move house and relocating my "brewery" room to the opposite end of the property. All 3 kids (young adults) either moved out or back into our home, which kept me fairly busy.
d. Throughout this period of downtime, I continued to brew where possible, but I only used the other 4 FVs. Due to the physical size of the Cooper's FVs I just didn't use them until more recently and I now realise that the infection I'm having issues with presently has been in these 2 FVs since May last year.