ETA 955 412 analog quartz runs very slow



What would be the symptoms of a poorly connected battery in an ETA 955
412 analog quartz watch?

Daughter got one at a garage sale. Not running. I fitted a new
battery. It ran perfectly for two months. Then suddenly started to
lose up to 40 minutes in 12 hours. No jumping in the second hand, to
indicate a low battery - but perhaps that movement does not have that
warning.

Checked battery. Voltage as new.

Now I have put battery back. Watch is running fine for now.

Unless I have unknowingly put right some other problem by opening up
the watch, I can only assume that the battery was poorly connected to
the terminals, thereby causing resistance. Yet the terminals were
clean and in fact gold plated.

What would be the symptoms of a battery that was poorly connected to
the terminals?

Any other suggestions?
.