PKG_MIRROR_HASH is always wrong

I'm trying to update a package (tvheadend). I'm forced to clone the github repo for the sources, as there are no recent releases. To generate PKG_MIRROR_HASH, I use make package/tvheadend/check FIXUP=1. But when I make the pull request for OpenWrt packages repo, the hash doesn't mach the one calculated by the OpenWrt automated checks.

I tried it multiple times, even using different commits in the tvheadend repository as the new package version. The hash calculated on my system is always different from the one calculated by OpenWrt checks.

What could cause that?
How do I fix it?

As a workaround, I let the check fail, manually copy the hash that it generated into the Makefile, and push the modification.