Catalyst Item Stats Changed Item Stats Changed After Using Catalyst.
Why did it remove Leech from my DoTI S3 M+ gear after showing me that Leech was still on the gear but it was now a S3 leg tier piece, hitting confirm for the change took off the Leech.
This is something probably best meant for the Bug Report forum. The SFAs that monitor this forum - they’re not Devs who have any part of the design or who would necessarily have the answers you’re seeking. Best to report it there. Mind you, it is a one-way reporting forum. Devs don’t respond to players there. At best you can also try to tweet them @WarcraftDevs.
It may also just be a display issue, the leech stat not showing up with mouseover but perhaps still may be active. That would still put the issue over to the Bug Reports forum, so that the proper staff can take a look into it.
If you look at the dungeon loot table, leech is a guaranteed stat baked into the pants and is not the same as a randomized tertiary. It will not stay with catalyst conversion, just like the operation: mechagon speed plate gloves in shadowlands removed the speed when catalyzed. It should have displayed the leech would be removed via the conversion preview before you accepted the change, but I know it wouldn’t always be something you’d be looking for. 
When I used the voracious hide pants last season, I made sure to have set on the other four slots and leave them as is to retain the leech.
yeah but it did show that the leech was still on them and it was on them until I put them on.
Ah, it sounds like the bug is in the catalyst preview then in this case.