Thanks. Looks like the reason I could not reproduce it is that the manifest file in that repo is inconsistent:
$ pkg resolve
ResolverError: Unsatisfiable requirements detected for package MeshCatMechanisms [6ad125db]:
MeshCatMechanisms [6ad125db] log:
├─possible versions are: [0.0.1-0.0.6, 0.1.0-0.1.1, 0.2.0-0.2.1, 0.3.0, 0.4.0, 0.5.0, 0.6.0, 0.7.0-0.7.1] or uninstalled
├─restricted to versions 0.5.0 by an explicit requirement, leaving only versions 0.5.0
└─restricted by compatibility requirements with MeshCat [283c5d60] to versions: 0.7.0-0.7.1 or uninstalled — no versions left
└─MeshCat [283c5d60] log:
├─possible versions are: [0.1.0-0.1.2, 0.2.0-0.2.3, 0.3.0, 0.4.0, 0.5.0, 0.6.0, 0.7.0, 0.8.0, 0.9.0-0.9.1, 0.10.0, 0.11.0-0.11.2] or uninstalled
└─restricted to versions 0.10.0 by an explicit requirement, leaving only versions 0.10.0
so Pkg kinda has to re-resolve here which results in those updates.