The issue was stumbled upon during environmental tests of a new design, loosely based on iMX6 smart device board.
Reboot instead of poweroff is always initiated on all the 3 boards available with both 1.2 and 1.3 silicon revision, when the processor junction temperature goes above approximately 93C, judging by embedded sensor data in /sys/class/thermal/thermal_zone1/temp. Other than this the tests are Ok, below the threshold poweroff is Ok, too.
The issue persists if the poweroff command is issued by directly writing to SNVS_LPCR register with memtool utility.
During the attempted poweroff the PMIC_ON_REQ pin drops low for approximately 3.8ms then goes high and a normal boot executes. VDD_SNVS_IN voltage shows a small momentary drop, but stays above 2.9V. PMIC interface signals have no other connections.
Thanks for the suggestions,