Edit: I figure this is a trivial bug, but report it only to identify it and because it seems like it could be part of a larger parser thing. Unless it isn't a bug and is part of the specification...
<latex> command requires space between ; and ] in Activity(Beta) for box object in PlantUML version 1.2020.08beta10.
Examples follow.
Not working:
@startuml
:Ready;
: <latex>P(y|\mathbf{x}) \mbox{ or } f(\mathbf{x})+\epsilon</latex>]
:finish;
@enduml
http://www.plantuml.com/plantuml/png/SoWkIImgAStDuR8AIarCgRJcibAmoKaiIQsm2z2eh8d9JIp9I4ghhgZLL8Z9JSglg5R8BrAeLKZJG9BHZaajACxCoS-pqOVeZEMoIilCoopEiEPoICrB0PeE0000
Working:
@startuml
:Ready;
: <latex>P(y|\mathbf{x}) \mbox{ or } f(\mathbf{x})+\epsilon</latex> ]
:finish;
@enduml
http://www.plantuml.com/plantuml/png/SoWkIImgAStDuR8AIarCgRJcibAmoKaiIQsm2z2eh8d9JIp9I4ghhgZLL8Z9JSglg5R8BrAeLKZJG9BHZaajACxCoS-pqOVeL8ZbiahBpCiiph3cSaZDIm4Q3m00