Apparent inconsistency of MOSFET power rating in datasheet: SOA v. max power
The SOA for this FET shows an additional limitation (beyond the traditional Vds, Imax, Rds, and P lines), that is not commonly found in other MOSFETs: the lines under the 10 ms and 1 ms markers. The only possible reason for this limitation that I found appears on the last page of IXYS Power MOSFET Datasheet Parameters Definition:

"
These theoretical constant power curves are derived from calculation with assumption of essentially uniform
junction temperature across the Power MOSFET die. This assumption is not always
valid, especially for a large die MOSFETs. ...(more explanation)... Modern Power MOSFETs optimized for a switch-mode
applications were found to have limited capability to operate in the right-side bottom
corner of the FBSOA graph."

Note that this transistor is intended for switching, not for linear operation, so it is probably optimized for that task: it is not designed for operating for longer time in this lower-right corner of the SOA. Note also that for small times (&lt; 10us, as occur in switching) there is essentially no such lower-right-hand-corner limitation.

For comparison: the SOA of a random MSOFET (irf7146):