Performance condition disappeared - release v20.31

Where in particular do you mean?

Did it come on the screen during the activity as normal?

Is the toggle still on? (System > Performance Condition)

I have a screen set up with it on as one of the data fields , it is still there

On yesterdays run PC came on the screen at the time it normally does

I can see PC on the GC Graphs Web and Mobile for the activity

I upgraded to 20.31 today and got the performance condition notification during my run and I do see it in the charts in Garmin connect

Performance condition does not require adding a field to the activity to get a result on completion.
You may have changed the translation of the field if you use a language other than English in the watch system.
Try using a chest heart rate monitor.
Another option is to translate the watch to another language and then back to the native language.

I have the same experience. I also have a data screen with it on and I checked yesterday and it worked as expected (i.e showing after the respected initial time). I also get the PC notification and graphs etc