Cassidy struggled to seventh in the first of two FE races in Indonesia, failing to make much in-roads from 10th on the grid in his Jaguar-powered Envision car despite saving his final Attack Mode activation in the hope of a late charge.
With title rival Pascal Wehrlein winning the race after qualifying a strong third, Cassidy's lead in the championship has been slashed from 20 points to just two ahead of the final six races of the season.
The Kiwi's worst result since the Diriyah E-Prix in January followed a sudden drop in pace for Jaguar in Jakarta, with both the works team and the customer Envision squad struggling for one-lap and long-run pace at the 2.4 circuit.
Asked if the Indonesian weekend is turning out to be a case of damage limitation, Cassidy told Motorsport.com. "I think for sure. It's just not a good track for us, clearly. All four of us were clearly struggling. So to finish seventh today was quite a good result.
"That's just the pace that we had. We were 17th in FP1, 13th in FP2, 10th in quali, seventh in the race, so we maximised what we had."
Cassidy was the only driver in a Jaguar-powered car to finish inside the points at Jakarta after team-mate Sebastien Buemi slumped to 21st after nearly stalling on the grid and works driver Sam Bird took out Mitch Evans while they were running ninth and eighth in the race.
Evans was encouraged by the fact that Envision was able to extract more performance out of the powertrain than the factory Jaguar team, but conceded that there was no obvious answer as to why the British-based manufacturer is lagging behind its rivals in Jakarta.
"There was a reason [behind our lack of pace] but I don't know how to fix this," Evans told Motorsport.com. "Nick had a good race compared to us so I will try and learn from that.
"But generally, at the moment we are really struggling now since FP1. We don't know what it is. We tried many things.
"Sometimes you go to some races and you're just fast, you can do anything to do the car and the thing is fast. Here we tried a lot of things but we were just slow.
"You can see a clear trend of Porsches and DS, Maserati and Andretti. They were all quick from the first lap in FP1 so sometimes it's like that. We don't know why. We try to understand but it's not always super clear."
The Jaguar powertrain had proved the be the benchmark in the middle part of the season, with Evans and Cassidy winning the last four races between them to end a period of early domination for Porsche.
But the pendulum swung back in Porsche's favour at Jakarta with Wehrlein winning the race and Jake Dennis finishing second in the customer Andretti car, ending a rough patch for the German manufacturer that saw its factory team fail to even finish on the podium at Sao Paulo, Berlin and Monaco.
Bird said Porsche's turnaround in performance came as no surprise to him, given its rich pedigree and extensive background in motorsport.
"It's a massive brand," he told Motorsport.com. "They started the year as clear favourites.
"Okay, they had a little bit of a rocky patch, but they are a quality outfit and it was only a matter of time before they bounced back and they can be quick at any given track."
