Since 9.2 went live, it appears that LTs are no longer consolidating in the game mail system. Since mail is capped at 100, if someone in the guild pops too many LTs it could begin pushing older mail out of your inbox.
I have had this issue previously with upgrading too many pets at one time and have lost some really important items from my mail. These lost items are unrecoverable.
I’ll chase this up, cos I can see your other Guild task mail is consolidating without issues so I don’t know why this would have changed.
Last week I did not see any consolidation of LTs in my game mail. Usually, beginning with the 5th unopened LT they would begin to bunch together into one mail. One of my guilds typically pops a lot of LTs in conjunction with new mythic releases. At one point I opened 30 individual LT mails in my game mail (at one opening). This did not seem normal to me.
I should add I was saving my previous weeks PvP goal reward mail for use in the vault event, so I had to open each mail individually to prevent accidentally claiming it too soon.