When there are two adjacent spans with tts:textCombine=all, they are combined into a single span containing the text from both.
when debugging, it seems that _isd_element is undefined on both spans (not sure why), and so are equal.

It would seem prudent to replace (in html.js):

        if (first.tagName === "SPAN" &&
            second.tagName === "SPAN" &&
            first._isd_element === second._isd_element) {

with

        if (first.tagName === "SPAN" &&
            second.tagName === "SPAN" &&
            first._isd_element &&
            first._isd_element === second._isd_element) {

this may mean there are other times when spans are combined when they should not be?