See issue #2116. Code for handling end of month (method couponFirstPeriodDate) needed a fix. Fixed it, confirmed it covered the reported issue with no regression problems. Then added some extra similar tests to all the callers of couponFirstPeriodDate, and ...
One new test, in COUPDAYSNC, does not agree with Excel. It also does not agree with LibreOffice. It does, however, agree with Gnumeric, and with my (hardly guaranteed) hand calculation of what the result should be. So, I'm going with it (and have added an appropriate comment to the test data). I'm glad to discuss the matter with anyone more familiar than I with how this is supposed to work - those 360-day years are killers.