It seems that if an enemy would so much as occupy the same segment as a tib patch, they just auto-die.
I presume this is to cut down on calculations, but it's very immersion breaking, especially given they die before actually getting near said tiberium (in fact, their pathing would of had them completely miss it).

Biters take damage from nodes, not ore tiles.

This is to prevent trying to check the tile under every biter every second.

They shouldn't be dying immediately though...

It might just be because they're small biters and the damage is too great for them to handle.
Here's an image of mass death via tiberophobia:
https://images-wixmp-ed30a86b8c4ca887773594c2.wixmp.com/f/9d676854-ca16-4cea-9099-fd954e997e6d/deslwap-ffa4acc3-1a62-4ecc-965a-c303ea7bc614.png/v1/fill/w_1280,h_720,q_80,strp/smallbitersdying_by_blacktemplegaurdian_deslwap-fullview.jpg?token=eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJzdWIiOiJ1cm46YXBwOjdlMGQxODg5ODIyNjQzNzNhNWYwZDQxNWVhMGQyNmUwIiwiaXNzIjoidXJuOmFwcDo3ZTBkMTg4OTgyMjY0MzczYTVmMGQ0MTVlYTBkMjZlMCIsIm9iaiI6W1t7ImhlaWdodCI6Ijw9NzIwIiwicGF0aCI6IlwvZlwvOWQ2NzY4NTQtY2ExNi00Y2VhLTkwOTktZmQ5NTRlOTk3ZTZkXC9kZXNsd2FwLWZmYTRhY2MzLTFhNjItNGVjYy05NjVhLWMzMDNlYTdiYzYxNC5wbmciLCJ3aWR0aCI6Ijw9MTI4MCJ9XV0sImF1ZCI6WyJ1cm46c2VydmljZTppbWFnZS5vcGVyYXRpb25zIl19.FSIqICYdXdFYWNkI6v8sdippQ66RvQV1vJ8keLZk2do

It might just be because they're small biters and the damage is too great for them to handle.
Here's an image of mass death via tiberophobia:
https://images-wixmp-ed30a86b8c4ca887773594c2.wixmp.com/f/9d676854-ca16-4cea-9099-fd954e997e6d/deslwap-ffa4acc3-1a62-4ecc-965a-c303ea7bc614.png/v1/fill/w_1280,h_720,q_80,strp/smallbitersdying_by_blacktemplegaurdian_deslwap-fullview.jpg?token=eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJzdWIiOiJ1cm46YXBwOjdlMGQxODg5ODIyNjQzNzNhNWYwZDQxNWVhMGQyNmUwIiwiaXNzIjoidXJuOmFwcDo3ZTBkMTg4OTgyMjY0MzczYTVmMGQ0MTVlYTBkMjZlMCIsIm9iaiI6W1t7ImhlaWdodCI6Ijw9NzIwIiwicGF0aCI6IlwvZlwvOWQ2NzY4NTQtY2ExNi00Y2VhLTkwOTktZmQ5NTRlOTk3ZTZkXC9kZXNsd2FwLWZmYTRhY2MzLTFhNjItNGVjYy05NjVhLWMzMDNlYTdiYzYxNC5wbmciLCJ3aWR0aCI6Ijw9MTI4MCJ9XV0sImF1ZCI6WyJ1cm46c2VydmljZTppbWFnZS5vcGVyYXRpb25zIl19.FSIqICYdXdFYWNkI6v8sdippQ66RvQV1vJ8keLZk2do

Update: Can confirm medium biters are dying just as quick.

Do you have the Tiberium Patch Size or Tiberium Damage settings cranked up to the max? We damage biters out to around where we expect the field to naturally expand to, but when there are cliffs blocking the growth that expectation doesn't match reality. Some things you could change to make it less of a death zone are to lower your Tiberium Patch Size or Tiberium Damage settings or to use the setting to make Biter Immune to Tiberium Damage.
Like James said, unfortunately we had to use rough damage zones for biters to avoid losing UPS.

That would probably do it! Both value and damage were between 2x and 3x default IIRC, and cliffs and tib spread were on their maximum setting.