I am Polish and I can not confirm that. I felt a sharp decline in "smiles" after my move to Germany where your description fits much more. I see much more people smiling for no obvious reasons when I visit Poland from time to time. Something which is not perceived as something else but friendliness by my German SO though while I've witnessed Germans being perceived as very cold by US Americans for the way they are.
I've been also smilingly welcomed by Russian friends even though they may smile less on the average. I haven't been to Russia yes so I can't tell. Maybe they are just well assimilated here.
Maybe it only is all those fake smiles you get from the US service culture which is so over the top that everything else becomes nuanced.
That makes sense. My main context for comparison is US vs PL, and there's a largish difference between strangers and in public or service people and a much smaller difference with friends and family. If you start talking to a stranger in the grocery store in the US because you were both reaching for the same milk you might get a very big smile. I would never expect such an exaggerated reaction in PL, just a small nod or pardon me.
Also, service people are not supposed to be fake smiling, we are actually expecting their emotional labor on top of the labor of their job. They are supposed to be cheering us up with their genuinely good attitude and "changing someone's day for the better" with their smile. It's all pretty exhausting.
I've spend a decade working for an F500 US company here in Germany. The amount of bad news delivered with a fake smile was staggering. I lead to pure disgust within the German employee bubble making it actually stronger and the news worse. In the end I've been fired with one of those and some phrase along the "let's stay friends and meet again" line ;)
I was always quite surprised that there seems to never have been any online course teaching people who came over those basic things as they we online courses for everything else.
I mean, you don't expect us to just hand over all our dark secrets like that in an e-learning module, do ya? The service people are supposed to mean it (if they're big enough suckers), but you can be more two-faced as you climb the ladder. I'd describe American business culture as "exploitative backstabbing bloodlust with a smile and an optimistic mission statement".
I've been also smilingly welcomed by Russian friends even though they may smile less on the average. I haven't been to Russia yes so I can't tell. Maybe they are just well assimilated here.
Maybe it only is all those fake smiles you get from the US service culture which is so over the top that everything else becomes nuanced.