Ulalek, Fused Atrocity is not triggering correctly. Ulalek's ability is a triggered ability that will trigger whenever you cast an eldrazi spell. Per Magic the Gathering comprehensive rules 11 08 2024 rule 603.2 "Whenever a game event or game state matches a triggered ability’s trigger event, that ability automatically triggers."

However this is not happening, the client is, for some spells, adding their CAST TRIGGERS to the stack and resolving them before adding the Ulalek trigger to the stack. Per Magic the Gathering Comprehensive Rules 11 08 2024 rule 117.2a Triggered abilities can trigger at any time, including while a spell is being cast, an ability is being activated, or a spell or ability is resolving. (See rule 603, “Handling Triggered Abilities.”) However, nothing actually happens at the time an ability triggers. Each time a player would receive priority, each ability that has triggered but hasn’t yet been put on the stack is put on the stack.

You can re-create this bug by casting Sowing Mycospawn, Writhing Chrysalis, Spawn-Gang Commander, Drownyard Lurker, and Warped Tusker with Ulalek, Fused Atrocity on the battlefield. Arena will always resolve these cards cast triggers before placing the Ulalek trigger on the stack