Nadal at Monte Carlo, lost in 2003 to Coria, won 46 matches in a row from 2005-2013, winning the title 8 years in a row from 2005-2012 and then reaching the final for a 9th consecutive year in 2013, where Djokovic beat him. Nadal won Monte Carlo another 3 times in a row from 2016-2018, 11 titles in total.
Nadal at Barcelona, lost in 2003 to Corretja, won 41 matches in a row from 2005-2009 and 2011-2013 (8 titles in a row, as he didn't play in 2010). Nadal lost in the quarter final to Ferrer in 2014.
Of course, Nadal went to 31-0, 70-1, 105-2 and 112-3 at the French Open, before ending at 112-4. The 31 in a row isn't Nadal's longest match winning streak at the French Open. Nadal had a longer French Open match winning streak at either 33 in a row or 35 in a row (depending on whether you count it as 2017-2021 or 2016-2021). Using traditional methods, it's 33 in a row, because a walkover loss ends any streak. And his longest streak was 39 wins in a row, from 2010-2015.