Seasonal Infused Stats on the items only last a duration of the current season. This is not a modding issue; it's how the game is designed with ever rotating seasons. Every season the game is introducing new content and this new content gets removed at end of the season but is replaced by a new system or content.
Season 24 (Etheral Infused)
Season 25 (SoulShard Infused)
This means after the season ends the infused stats on the items could be altered, changed or removed as the game won't be able to recognize last seasons affixes.
You normally wont lose the item entirely after season ends but just the seasonal infused stats will be changed reflecting the new patch.
We value our supporters so ordering a Seasonal Infused Stat items will allow the buyer to claim a 25% discount when the next Season season starts and new Seasonal Infused items are introduced if their previous items were altered or changed by the game. This can be claimed once every 2 season cycle. Buy Season X1 @ full price, redeem 40% off season X2.
In patch 2.7.2 they have introduced a new system called SoulShards that work like ETH items in a way however these shards stats are a lot weaker than ETH stats.
None Seasonal Infused items are not affected by duration of an ending season.