What does it mean when an ETF has a negative turnover rate?

This article might answer your question. It seems to be due to the formula used for UCITS funds:

Back in the day, when it was still requirement, UCITS III used the following formula:

Turnover = [(Purchases plus Sales) – (Cash in plus Cash out)]/Avg Net Assets

The main issue with this formula was that it produced a negative turnover ratio where the cashflows exceeded the total trades, and that clearly makes no sense. It would also inflate the ratio when the fund performed many ‘paired’ trades (ie, buying one instrument on the back of selling another).

The SEC uses a very simple formula:

Turnover = Min(Purchases;Sales)/Avg Assets included in numerator