Question: Tabula rasa (table redefined)

Something happens when I redefine an indexed value of a table, but I'm not sure what.

Here's the situation before the definition.

eval(WhateverYouNeed["calculations"]["structure"]["connection"]);

TABLE(["Cutright1" = "false", "graindirection1" = 90 Unit(arcdeg), "Cutleft2" = "false",  "lengthleft2" = 500 Unit(mm), "Cutrightsteel" = "false", "lengthleftsteel" = "false", "lengthleft1" = 350 Unit(mm), "lengthright2" = 500 Unit(mm), "connection_1" = "Timber", "lengthrightsteel" = "false", "connection_2" = "Timber", "Cutleft1" = "false", "Cutleftsteel" = "false", "graindirection2" = 0, "graindirectionsteel" = "false", "Cutright2" = "false", "lengthright1" = 150 Unit(mm), "connection_InsideLayers" = 1])

I then try to redefine one specific entry of the table.
WhateverYouNeed["calculations"]["structure"]["connection"]["connection_1"] := GetProperty("ComboBox_connection_1", 'value');

WhateverYouNeed["calculations"]["structure"]["connection"]["connection_1"] := "Timber"

Which gives this new result.

eval(WhateverYouNeed["calculations"]["structure"]["connection"]);

TABLE(["connection_1" = "Timber"])

The rest of the indices and entries of the "connection" table is gone.