I'm a US citizen traveling through Europe visa-free. I started my trip in Spain and got an entry stamp there. Later, I took the IDBUS from France to the UK and got an exit stamp from France and an entry stamp from the UK. (They made us all get out and go through two separate passport checkpoints.) Yesterday I took the night IDBUS bus back from the UK to Amsterdam. This time, we didn't have to get off the bus for passport checks: a UK official and a French official got on the bus and quickly checked everyone's passports, but no stamps were given. The last stamp in my passport is now my entry stamp into the UK, even though I'm in the Schengen Area again.
I'm confused: at the very least, shouldn't I have gotten an entry stamp from the French official? Is this common? What does this mean in practice? Will the burden of proof be on me to show that I haven't overstayed my 6-month period in the UK or my 3-month period in the Schengen region whenever I go through another border checkpoint?