I can't encode a acitivity-diagram to svg format to occur the Ecoding error using PlantUML V1.2022.2.
Without '-svg' option, the same activity-diagram was successfully encoded to png format.
And also, the same activity-diagram was successfully encoded on PlantUML Web Server.
PlantUML Web Server
The follwing bug has not been completely fixed.
Encoding error with nested diagrams - PlantUML Q&A