Bridge textures not showing up is also visible in that snip I posted; it looks at items, then at buildings, then all tiletypes, then applies first override it finds for a given tile and skips the rest. It then still displays terrain overrides underneath because it then redoes the map check again, word-for-word, in another function below it in same file...which, whoops, doesn't have my water/hidden fixes - well, that explains why numbers showed up when items and water shared a tile^^;;
Worth mentioning since I don't think it's explained anywhere: consequently even items by themselves have a bug, in that if there are overrides for a tile, it looks at all matching item groups, oldest items and overriddes first, until it finds an item on that position. Vanilla DF displays piles of items in "youngest first" order, and the pile of items can have not-overridden item on top besides.
Minor ex: This means a full set of clothes will always have same item showed on top, no matter production order.
Major ex: If a tool shares a raw tilenumber with another item (ex, quire and metal bars), and you have a tool (say, wheelbarrow) on a tile with younger metal bars, then it will display the override for quire.
As for vermin, that's really vanilla bug, though luckily the tile they should have is still loaded in game memory. Pissed of Nopal badly (though the record of that is gone as he or Toady erased his account).
I did play around with overrides for engravings toggled on, but it would involve writing double the amount of TWBT overrides that I already have written. Also the possible bugs would make it not worth it.
That's why I did sentient corpse overrides (which also needs altered twbt), tbh - lot of engravings are about creatures, and mostly-transparent text image looks acceptable on ground (though getting actual creature tile from raws would be possible), but horrible on walls, and walls can have so many shapes.