Thanks for all the suggestions, I am very grateful to all those who answered.
After further testing, I believe those who said it was a synthesizer thing were right in this case. Here are my findings:
I double checked and Word Wrap was selected in Notepad.
I tried Say All once more and the reading stopped after the first three or four lines again.
I asked it to spell the first line as suggested, with insert+up arrow twice quickly, and it only reported the first character of the first line.
I tried the same thing with two other different files, same behaviour.
I opened a web page and asked it to spell a line, same behaviour.
For all these tests I've used the Code Factory vocalizer synth, the one that I had intended to use for book reading.
Now, as soon as I switched to Eloquence, NVDA went back to behaving normally. Both "say all" and "spell the line" work perfectly now, in Notepad, Word, and Chrome.
Therefore, I have another question if you don't mind:
Should I send an e-mail to CF about this? How can I describe the problem to them? I am not a programmer, so I fear my description won't be detailed enough, and I might not be able to give them the necessary elements.
Many thanks once more, and a nice end of the week to everyone.