We've tackled this once is bug 2983. In section 3.2.2 upd:applyUpdates, Semantics, bullet 3, we now say,

"If, as a net result of the above steps, the children property of some node contains adjacent text nodes, these adjacent text nodes are merged into a single text node. The string-value of the resulting text node is the concatenated string-values of the adjacent text nodes, with no intervening space added. The nodeid of the resulting text node is implementation-dependent."

In my opinion, this is a somewhat loose statement. One implementation might create a new text node, with new identity, as a result of the merge, and the disconnect the original text nodes from their parent node. Another implementation might add the content of the second text node to the first text node, and then disconnect the second text node from its parent node.

We have reviewed this issue: comment #1 answers the question stated in the bug description.  No further changes are necessary.