I provide the download link in server.properties on the line resource-pack=, and when first logging into the server, all works as intended up to this point- The pack is downloaded and correctly displays on the client.
However, if I ever update the resource pack (Which often results in the pack having the same download URL), it will never seem to update on any client. (I am disconnecting/reconnecting to the server, and even restarting both the client and server.)
After trying to get this to work for a while this broke entirely after removing the resource pack file in /.minecraft/server-resource-packs/ in an attempt to brute-force a re-download.
I ended up receiving no resource pack with the log file only telling me the nondescript error of "Invalid resource pack" (It is entirely a valid resource pack, I can load it onto my client just fine)
After enough brute force restarting everything multiple times, forcing the URL to change, and waiting a few hours it eventually worked without any reasoning behind why, as I had already tried all of these multiple times until it just suddenly 'worked' again.
This behavior is wildly inconsistent and confusing.

What I think is going on is one or multiple of the following:
• Minecraft 'remembers' it already downloaded a resource pack from the server and simply doesn't check at all if it's different
• It will only check for a URL change
• It will only check again if a certain amount of time has elapsed
• It will only check again if the corresponding file in /.minecraft/server-resource-packs/ does not exist (very likely in combination with one of the other possibilities if it does check this at all)
• It will only check again if the server or client has restarted
How this should behave:
• Upon joining, the player should receive the resource pack (regardless if they have before) if it has changed.
If nothing else, there should at the very least be a way to force the game to re-download the server's resource pack should it somehow fail.