My intention was not to take anything away from the achievements of GrapheneOS.
Coming back to the question, only security updates for Mainline modules are documented in ASB while features updates aren't documented anywhere. So, if a new feature is rolling out via a Google Play System Update, then chances are GrapheneOS users aren't getting those. This is just Google being notorious.
There is, however, a way suggested by Mishaal Rahman to get feature updates too:
Somewhat recently, though, Google started publishing AOSP tags for each Mainline module release after the completion of a GPSU rollout. [By maintaining a changelog between tags,] It's possible for custom ROMs to compile those separately and then include them as prebuilts into their own projects.
Is it unreasonable to request for the same features that GMS Android users are getting?