Every ISS crew member must always have a seat on a lifeboat available at any time. I'm pretty sure this is dictated by ISS Flight Rules, and I'm pretty sure there are members here that have access to those and can confirm that. Unfortunately, unlike Shuttle, the Flight Rules are not published.

If you undock Crew Dragon without its crew in it, then its crew will have no lifeboat available during that time, thus violating Flight Rules.

There was an interesting situation in 2023 where those lifeboat rules were tested: the Soyuz MS-22 capsule had sprung a leak in its coolant system, and there was concern that the astronauts would be subjected to high temperatures during the return flight. Therefore, it was decided to push the Soyuz MS-23 mission forward and turn it into an uncrewed mission.

However, this meant that cosmonauts Sergey Prokopyev and Dmitry Petelin, and astronaut Frank Rubio would have had no lifeboat for two months until the arrival of Soyuz MS-23.

NASA decided that Frank Rubio's seat liner would be un-installed from Soyuz MS-22 and jerry-rigged to the cargo compartment of Crew-5's capsule Endurance. While Crew Dragon always flies with a crew of at most 4, it was originally designed for 7, therefore, its environmental control and life-support systems would easily be able to handle a crew of 5. Crew Dragon's water landing is not nearly as violent as Soyuz's land landing, so not having a proper seat for Frank Rubio was deemed acceptable.

Roskosmos decided that Soyuz MS-22 was probably safe enough for an emergency return, especially with the reduced heat load of only 2 cosmonauts, so it served as lifeboat for Sergey Prokopyev and Dmitry Petelin.

In the end, none of that was needed. After the arrival of Soyuz MS-23, the three seat liners were moved from Endurance and Soyuz MS-22 to Soyuz MS-23, at which point, the normal lifeboat situation was re-established.

Soyuz MS-22 returned uncrewed. To make its return as easy and fast as possible, the ISS was pitched 90° and the Soyuz's thrusters were fired longer than normal, reducing the return journey to only 55 minutes. The temperature reached 50°C, which was lower than the worst-case estimates, and would have been survivable.

Every ISS crew member must always have a seat on a lifeboat available at any time. This is dictated by ISS Flight Rules, as confirmed in a comment by former Operations Controller Doresoom. Unfortunately, unlike Shuttle, the Flight Rules are not published.

If you undock Crew Dragon without its crew in it, then its crew will have no lifeboat available during that time, thus violating Flight Rules.

There was an interesting situation in 2023 where those lifeboat rules were tested: the Soyuz MS-22 capsule had sprung a leak in its coolant system, and there was concern that the astronauts would be subjected to high temperatures during the return flight. Therefore, it was decided to push the Soyuz MS-23 mission forward and turn it into an uncrewed mission.

However, this meant that cosmonauts Sergey Prokopyev and Dmitry Petelin, and astronaut Frank Rubio would have had no lifeboat for two months until the arrival of Soyuz MS-23.

NASA decided that Frank Rubio's seat liner would be un-installed from Soyuz MS-22 and jerry-rigged into Crew-5's capsule Endurance. While Crew Dragon always flies with a crew of at most 4, it was originally designed for 7, therefore, its environmental control and life-support systems would easily be able to handle a crew of 5; this also means there is enough space on the cargo pallet behind the four seats, where the other three seats would have been. Crew Dragon's water landing is fairly benign, so not having a proper seat for Frank Rubio was deemed acceptable – remember, we are only talking about an emergency return, where different Flight Rules apply and NASA is willing to accept more risk.

Roskosmos decided that Soyuz MS-22 was probably safe enough for an emergency return, especially with the reduced heat load of only 2 cosmonauts, so it served as lifeboat for Sergey Prokopyev and Dmitry Petelin.

In the end, none of that was needed. After the arrival of Soyuz MS-23, the three seat liners were moved from Endurance and Soyuz MS-22 to Soyuz MS-23, at which point, the normal lifeboat situation was re-established.

Soyuz MS-22 returned uncrewed. To make its return as easy and fast as possible, the ISS was pitched 90° and the Soyuz's thrusters were fired longer than normal, reducing the return journey to only 55 minutes. The temperature reached 50°C, which was lower than the worst-case estimates, and would have been survivable.