So far so good on the most precocious and vigorous of the GTS runners planted under the nanking cherry in January '23
Tuber is firm and vital-seeming. Frosts have been down to - 4F/-18C this year. Not sure how much tuber is underground but overwintering perennials frequently get pushed up by frost heave. I plan to leave it alone for now and keep tabs on it. If it produces seed, and if it starts flowering and seeding early and abundantly enough, I may see about thoughts and feelings on sending some in for growers to try for fall seeds. Though since this might mainly be off interest to growers in zone 7 and lower (I’m in 6), that’s a lot of ifs. Always next year’s mix though 
Assuming it’s indeed still alive I credit at least these following things:
- vitality of the original seed (genetics factor in here)
- embryonic exposure to freezing temperatures while in conditions of the final planting site
- a sheltered location
- shelter provided by an extremely cold hardy perennial that had been there several seasons