I think you're right... I've tried Fairies on full revealed Faron Woods with over 50 buildings and over 100 units but didn't built any additional Sundials, and changed seasons through several loops, game will only freeze for about a second and won't totally freeze like the matches having multi Sundials
So if you set a limit for Fairies of Tarm to can only have one Sundial, but add a building like Gerudo's Oasis that can generate large territory and without the ability of changing season, will solve this fatal freezing problem?
And I found another bug: the All Four Colors tech at Fairy Fountains, that should let you train Fairy Drove with all season efforts after researched it, will only function in spring... in other seasons and the Fey Gate in autumn, will only let you train regular ones even if you researched the tech other than spring