If it’s something portioned directly in front of the customer, it can happen- this is definitely a phenomenon in our work canteen. One of the staff consistently serves noticeably bigger portions to all the male customers, especially the younger ones, than she does to the female customers.
On a similar note, a friend of mine spotted this happening when ordering a bottle of wine for the table when out with her husband here in the UK. The sequence typically goes: server comes over, gives customer a small glass to try it, asks if it’s OK, then top that up to a proper glass, pours remaining customers a glass, leaves bottle on table. Being that sort of person (a research scientist at the time) she decided to check if her suspicions of bias were based on reality, as they ate out a lot at different restaurants. Every time, she photographed the two newly poured glasses next to each other and recorded who was asked to check the wine and who was given the bill.
In the time she kept up with doing it, which went on for about a year, she never once got the larger serving and only one server poured in a way that wasn’t visibly uneven, who was also the only one who poured both a sample for approval. The rest of the time the server poured one sample and the majority of the time (I can’t remember the proportion, but it was something like 80% of the time), gave it her husband for approval before pouring hers, even though she placed the order every single time. It was a while back, but she was sharing it on social media with pics of the wine glasses. Some of them were fairly close, just hers slightly lower, some were ridiculously different.
Her husband also got presented with the bill every time they gave it to one person, rather than placing it on the table.
They’re both from mainland Europe originally, and this did not happen on trips to visit family, just in the UK. The one server who poured evenly was also Dutch.
So yeah, I won’t argue that there can be bias in restaurants, but as everyone’s pointed out, this can only apply at the customer-facing bit, cos the kitchen normally doesn’t know. Maybe in a small cafe where they can see the customers it could happen - I have worked in a place like that and the chef was making stuff knowing who was eating it, because it was mostly regulars.