Thanks! I can replicate the issue and I think it might be because it does not recognize the 々as a kanji, but will need to debug it to fix it properly.
I do want to note that for now you could also solve your problem differently by doing the following:
先々月[せんせんげつ]
You can combine the furigana into one bracket for the whole word if the word consists of kanji only (if there is okurigana you’d have to split it up before and after it.
Another small offtopic note that might be a bit more convenient for you is that you can use full-width brackets (Japanese width) instead so you don’t have to switch keyboards/IME to enter the furigana syntax. Like so:
先々月{せんせんげつ}
In any case I’ll put it on the prio to-do list and fix it asap