The following advisory appears near the top of the page describing the upgrade process, https://puppet.com/docs/puppet/latest/upgrade_minor.html:
The order in which you upgrade components is important. Always upgrade Puppet Server and PuppetDB simultaneously, including the puppetdb-termini package on Puppet Server nodes, and always upgrade them before you upgrade agent nodes. Do not run different major versions on your Puppet masters (including Server) and PuppetDB nodes.
Further down the page, however, the first sentence in the "Upgrade PuppetDB" section is
PuppetDB can be upgraded independently of your master and agent nodes.
So which is it? PuppetDB and Puppet Server must be upgraded together, or they can be upgraded independently? I suspect that it depends on the details of the upgrade in question, but those dependencies should be documented.
My educated guess would be that the first passage is meant to apply only to upgrades between major versions, whereas the latter is about upgrades within the same major version.