We just upgraded to TeamCity 2021.1 and we noticed that build chains no longer behave the same way they did in 2018.1. It seems that 2021.1 sometimes triggers a full builds of the entire chain, but in 2018.1 it seems TeamCity would use artifacts that were acceptable.
Consider this scenario: Build Library and Build App are my build chain with snapshot and artifact dependencies. Build Library has checkout rules to limit what it sees in the repository. The whole build chain was built successfully, then I make a commit that is outside of the checkout rules for Build Library so it should only trigger Build App. I then trigger Build App but Build Library also triggers even with no changes (and TeamCity even shows me no changes).
In 2018.1: this would only trigger Build App and it would use the Build Library artifacts that matches the source from the last build.