Why arco Text changes across scores?

Using MS 3.0.5 on Windows 7.
I started a score which includes string instruments, set them to pizz. using the pizz. text from the Text pallete (very handy). When I tried to go back to arco with the arco text, it didn't work. I went to text properties and found three possibilities: normal, pizzicato and tremolo. Normal was set, but nothing normal happened (see Test_arco1.mscz, bass part, sorry the names are in Spanish, it's the second from bottom). Then (see viola part), I just did as if I selected normal (even if it seemed to be already selected), and it worked fine.
Then I tried to write an example from scratch (Test_arco2.mscz), and strangely (or Murphily!) it worked as it is normally expected. When I examined the text properties, I found that the three possibilities were arco (instead of normal), pizzicato and tremolo. Arco was selected when the arco text was inserted, and it worked without any further action on my part.
Any idea of what is going on?