The following happened in a 4-player game of multiplayer commander. Screenshot is attached for reference.

Repro steps:

1) Active player (P1) attacks P3 with Glissa, the Traitor and Solemn Simulacrum, and attacks P4 with Gorm the Great and Virtus the Veiled.
2) During declare blockers step, P3 attempts to block only Solemn Simulacrum. This should be a legal block as Gorm is attacking P4.
3) Actual: No configuration of blockers allows P3 to assign blockers, every configuration is given a warning: "Illegal blocking - Gorm the Great must be blocked if able."
4) Expected: P3 is able to assign Ramunap Excavator as a blocker to Solemn Simulacrum and declare no other blockers.

Additional information:
At the beginning of the declare attackers step, P3 controlled Grid Monitor, controlled by P4. P4 controlled a 3/3 green Elephant token. After attackers were declared, P4 activated Homeward Path, and Active Player responded by activating Walking Ballista 3 times to deal 3 damage to the elephant token. The elephant token died, triggering Glissa, returning Walking Ballista. Homeward path then resolved, giving P4 control of the Grid Monitor.