Issue:

1. The player has a timer for a boss with a known shard id.
2. Some other player who is on WBT < v1.10 shares a timer for the same boss. That timer will not include shard data.
3. For the receiving player, this will create a timer without any shard, but with latest version, so you'll have timers like this:
   • name=Nalak, ver=1.10, shard_id=nil
   • name=Nalak, ver=1.10, shard_id=123
4. Bug: Sometimes the wrong timer will be found, e.g. player shard is known, but the shard=nil timer is found, because it uses findfirst()-ish filter. And filtering always keeps shard_id=nil. When trying to share the timer, it will either result in the sharing player getting a message that it has expired (if it's the shard_=nil and it has expired), or the wrong one will be reported, or the correct one will be reported, all depending on findfirst()-ish behavior.
5. Bug: The GUI will highlight shard=nil timers, even if player shard is known.

Should relate to both these reports:
https://www.curseforge.com/wow/addons/worldbosstimers?comment=339
https://www.curseforge.com/wow/addons/worldbosstimers?comment=341

Fixed as of 57c525a

Timers without a shard ID (received from another player sharing) can no longer be shared.