nanopb doesn't follow https://semver.org/. It uses four digit versions and has only used major version 0 which in semver implies any update can be breaking.
The CocoaPods minor version is calculated from the nanopb version with the
formula:
minor * 10,000 + patch * 100 + fourth
The CocoaPod major version is not 0 because some CocoaPods incorrectly published with floating dependencies allowing updates to any 0 major version. Other major version updates may be done because of iOS support versioning changes even if the underlying nanopb library is unimpacted. For example, the major update from 1 to 2 is done to bump the minimum supported iOS version to 9 because Xcode 12 no longer supports iOS 8 and generates build warnings for nanopb client apps.
The CocoaPods patch version should be used for any podspec or other packaging updates.
While nanopb tries to follow a variation of semver and updates the fourth digit
for non-breaking patch updates, the policy is not enforced for binary
dependencies which can break when source dependencies don't, like the #define
changes in 0.3.9.4.
The pod instead looks for sources from the podspec's source attribute.
Publish this pod with --allow-warnings to address a warning caused by
the podspec's version attribute not matching a tag in the source-of-truth repo.