I am really frustrated about this, because I've tried to write down a calendar about my RoG and Hamlet seasons so I don't have to jump in the Skyworthy to know what's going on in the other side. Turns out RoG seasons correctly follow the 20-16-20-16 pattern (interestingly enough, I found no information that in Together it is 20-15-20-15 instead) but when I've tried to figure out the Hamlet seasons it seemed to be inconsistently oscillate between 10 and 11. I've looked it up and found no definitive answer about it, although most people seemed to believe the seasons are 11 days long. Well, I'm starting my 4th Hamlet year at day 96, which just doesn't add up with 33-day nor 30-day years.
I've started this world in RoG and then around day 70 or so I started a new separate Hamlet world, which I immediately merged with the RoG one in order to avoid starting in the Aporkalypse. Unfortunately, I've only realized the short seasons after I rebuild the Skyworthy in Hamlet at around day 134 (day 63 in Hamlet), so I'm quite unsure whether the world hopping is behind it or not. What I know is, that since I've started to use the Skyworthy on almost a daily basis, it got more and more obvious that the Hamlet seasons are out of synch.
I've not linked to Shipwrecked yet and I only use a few mods, like Geometric placement, Crocpot recipes, Finder redux, Simple healthbars, Combined status, Minimap and Food values.
I presume, using the Skyworthy to travel between RoG and Hamlet worlds at the turn of the Hamlet seasons may be the source and so a way to reproduce it. Unfortunately, I haven't been able to purposely reproduce it yet.