Thomas’ and Thomas’s are the same thing.
Thomas’ and Thomas’s are the same thing.
https://grammar.collinsdictionary.com/easy-learning/what-are... https://www.sussex.ac.uk/informatics/punctuation/apostrophe/...
However, it's English: there isn't just one rule, another rule can also be valid and might be the one you're familiar with on a day to day basis. That doesn't mean any other way to say or write the same thing is wrong, it's just a pattern you never saw. Like someone going "lol snuck isn't a real word, it's sneaked!" and then you hand them a dictionary and they learn something new about their own language.
[1] https://www.ox.ac.uk/sites/files/oxford/media_wysiwyg/Univer...
Um... what? Pronouncing a possessive suffix with /səs/ isn't valid anywhere. The only possibility is /səz/. Same goes for the plural suffix.