Game 85 was a Benko Gambit, white started with a pawn up and a passer on the queen side. Stockfish had an small eval advantage after the opening and the action was focused on the open queen side. Houdini brought all its pieces there to block the white pawns. After a knight blocked the b-file and the queens were exchanged the evals started to fall to 0. After opening the king side Stockfish got the pawn back and created passers in the center. Eventually Houdini had to give up its passer to stop the black pawns, the engines continued to exchange until reaching a drawn rook ending.
In game 86 Houdini took a different approach. It gave up the pawns advantage and the passer to open up the queen side for its queen and rook. This led to many exchanges and a RN vs RN position on move 30. Houdini's eval advantage was gone a few moves later. Stockfish exchanged the rooks and the pawns were quickly captured, when the last white pawn was gone Houdini could not win with only a knight. Two Benkos, two draws.
Game 87 started with both kings in the center and with black's king side open. Stockfish captured two pawns in the center but its king was stuck and could not castle while Houdini's king castled long. Houdini attacked through the king side with a QN combination, leading to exchanges and a RRB vs RRB position. Stockfish was still a pawn up but Houdini's rooks were more active. After exchanging a pair of rooks the extra pawn was not enough in an opposite color bishops setting.
In game 88 both kings remained in the center and queens were exchanged early. Houdini was a pawn up and Stockfish attacked through the king side, leading to a series of exchanges and a rook ending by move 28. Evals fell quickly to 0 and the game ended in a draw after most of the pawns were gone.
Game 89 started with black a pawn up, the queen attacking and pieces in awkward positions. The evals stayed low throughout the game, both kings did not castle but were relatively safe once the queens were exchanged. Only RRB vs RRN remained after move 28, and after a few pawn exchanges Stockfish ended the game with perpetual check.
In a similar position in game 90 Houdini offered a bishop sacrifice to open up the king side but Stockfish declined. After a series of exchanges Houdini was a pawn up in a double rook ending on move 31. The engines eliminated the queen side pawns, then exchanged a pair of rooks. The remaining rook ending was a draw but it took about 50 moves for the engines to agree.
Game 91 started with a closed Spanish opening, most of the pieces remained on the board with pawns mostly locked. Stockfish had a small eval advantage, perhaps a small space advantage and two knights that are stronger in closed positions. Houdini's bishops were quite miserable.
Stockfish's eval gradually increased though it was not clear why at first. Houdini exchanged knights, leaving Stockfish with one while Houdini had none. There were many weak pawns, both black and white, in particular the black b4 and g4 pawns could not be defended. After exchanging queens the black king side pawn line was broken and evals were over 1.5.
Taking its time Stockfish captured the b and g pawns, exchanging a rook pair in the process with RBN vs RBB remaining. Stockfish then sacrificed two pawns and created 3 passers across the board.
Houdini also had two passers but the white pawns were a more immediate threat. Houdini could not defend against all three without losing material. Stockfish squeezed a win out of this closed position, it is at +9.
In game 92 the engines exchanged more pawns and the position was more open. After opening the queen side both engines had a passer there. The engines exchanged queens and Stockfish gave the queen side passer and gained one in the center. Stockfish's eval at this point fell to 0, Houdini kept a small eval for a while longer. The game reached a RN vs RB position, Houdini kept one last pawn but it was not enough for a win.