Started in SW, debug spawned Seaworthy, went to RoG (right away on day 1) in order to sync the world timers and because I wanted to go from RoG > SW not the other way around. Now every time I reloaded the game, it would reset to day 1 in the RoG world.
At about 14 days of Spring left (standard season lengths, and I started in Autumn so 20 days of Autumn + 15 days of winter and then about 6 days of Spring or about 42 days), I finally got all the parts to craft the RoG version of the Seaworthy to go back, was back in the SW world at day 42, but it said 21 days of Monsoon season left (and this held true in the sense that the season did last that long, even though it isn't supposed to actually last that long by default, and it should have been deeper into it by day 42 anyways).
Sometime during Dry Season, went to Volcano, after this (I THINK THIS WAS THE TRIGGER AT LEAST), reloading in the SW world also would reset to day 1, while being in the volcano gave the 'true' date in terms of days. (The order of this event might be flipped with the next one, I didn't keep detailed notes)
Sometime around day 70 (hard to keep track of because of the day bug), went back to the RoG world again, said day 48, 11 days of summer left (and dragonfly spawned). And going back to SW, spending time there, did not advance the 'days left on summer' at all. However summer still seems to be continuing on 'schedule' (or at least the messed up schedule it currently has), as long as I am in the RoG world.
TLDR: Day tracking is COMPLETELY broken (and this does affect hound waves, full moons ect, so it has actual gameplay effects, but doesn't seem to affect season progression, but season progression itself seems broken when transitioning worlds). From further testing with console in new games, seems like it's a case of 'reseting to days ORIGINALLY spent in that specific realm' or something like that
Somewhat unrelated: Appeasing the volcano fully still caused an eruption after I exited; seems to only delay eruption INSIDE the volcano rather then outside as well. Seems like living IN the volcano is safer then outside ATM during Dry Season.
I don't even want to know what going into Caves/Ruins in the RoG world will do to the days tracker.
Probably going to keep playing this world, will update if I see anything else interesting (unless I restart because this is pretty annoying tbh as I hate going back to day 1 over and over).
Pretty much described above, although it's not 100% clear to me EXACTLY what causes the issue, but it has to do with world hopping via the Seaworthy and spending time separately in both 'worlds', and then saving + exiting and loading the game, which seems to cause it to reset to some kind of 'days originally spent in the other realm' or something; the bug is not very clear to me. Sorry that I can't be more help, but it's a confusing bug to me even with console testing I'm not 100% sure on all the details.