Based on Brachot 45b and Yerushalmi Brachot 5:4/40b, many communities have the custom of saying amen after several of their own brachot. One of the places where some add amen is at the end of the brachot after shema. In the evening, the series of brachot after shema consist of emet v'emuna אמת ואמונה (ending in ga'al yisrael גאל ישראל), hashkiveinu השכיבנו, and yir'u eineinu יראו עיננו (although some omit the latter, and some combine the last two into one bracha).
From what I understand, the amen must come at the end of the series of brachot (see Mishneh Torah Tefillah 1:16–18, Rashi, Rosh, Rabbeinu Yonah, etc on the gemara). Thus, one would expect this amen to come at the end of yir'u eineinu. However, in Minhag Roma as practiced now in Israel (see here, p 110 of the PDF, p 96 on the page), the amen comes at the end of hashkiveinu! Perhaps the intention is to separate between the Talmudic brachot (ga'al yisrael and hashkiveinu) and the Geonic bracha of yir'u eineinu, just like amen after boneh yerushalayim separates the Biblical brachot from the Rabbinic bracha in birkat hamazon?
I'm interested in literature on this point, and whether my guess is correct. In particular, I'm interested in discussions of whether to place amen after hashkiveinu or after yir'u eineinu, in contexts where both brachot are said independently (so, not where the latter is omitted or where it's combined with hashkiveinu, and not where amen is omitted entirely).