I think it depends on age groups. At the u11 & u10 level, its g to see 2 periods focused on development and I hven't seen a kid not play hard until 3rd period because it didnt "count." I have seen some parents cheer harder in the third
That said, this observation is at the u11, u10 level. It's a non-issue and the kids are just playing. I can imagine the older kids with a few years of "normal" games might act differently.